Getting Started 
Misconception/Error The student is unable to explain the significance of a point on the graph of the linear model. 
Examples of Student Work at this Level The student:
 Only states or demonstrates that (20, 11) is a solution of the equation.
 Describes the position of (20, 11) in the coordinate plane or states that it is a point on the graph.
 Provides an incorrect interpretation of the point.
 Restates the coordinates with units but does not explain their association in context.

Questions Eliciting Thinking Can you explain what the equation models?
What is the meaning of each variable in the context of the model?
What does it mean for x to equal 20? What does it mean for y to equal 11? 
Instructional Implications Review as needed:
 Independent and dependent variables and how functions that describe the relationship between them are represented by equations, tables, graphs, and verbal descriptions.
 The concept of a linear function and its graph.
 The slopeintercept form of a linear equation, y = mx + b.
 Solutions of equations in twovariables as ordered pairs of numbers.
Explain that the line of best fit models the relationship between the two variables, cord length and weight on the cord. Provide instruction on how to use a linear model to make predictions about the value of one variable given a value of the other. Ask the student to find the length of the cord when x = 20 and write the result as an ordered pair. Ask the student to explain the significance of each value in the ordered pair.
Ask the student to use the equation to make other predictions about the length a cord stretches when a certain weight is applied or the amount of weight needed to stretch the cord to a certain length. Ask the student to write each prediction as an ordered pair and clearly explain its meaning. 
Making Progress 
Misconception/Error The student is unable to explain the meaning of the yintercept. 
Examples of Student Work at this Level The student interprets (20, 11) by saying when 20 pounds of weight is applied, the bungee cord will stretch to 11 feet in length. However, the student is unable to correctly interpret the yintercept in the context of the model.

Questions Eliciting Thinking What is the yintercept? If you graphed it, where would it be located?
Which two variables are related by the model? Can you explain the meaning of the coordinates of the yintercept in terms of these variables? 
Instructional Implications Review the concept of a yintercept. Discuss how it is represented on a graph (as a point on the yaxis) and in an equation written in slopeintercept form. Explain that the yintercept is the yvalue (cord length) that corresponds to an xvalue (weight on the cord) of zero. Have the student write the yintercept as an ordered pair including units of measure, (0 lbs. of weight, 10 ft. of cord length). Guide the student to interpret the yintercept as an associated pair of values (e.g., “a bungee cord with 0 lbs. of weight on it measures 10 ft. long”). Then have the student consider whether this statement makes sense in the context of the model. If the statement makes sense, then have the student reinterpret it using other wording (e.g., “a bungee cord is 10 feet long when no weight is applied”).
Provide additional opportunities for the student to identify and interpret the yintercept in linear models. 
Got It 
Misconception/Error The student provides complete and correct responses to all components of the task. 
Examples of Student Work at this Level The student explains that:
 A bungee cord holding 20 pounds is predicted to stretch to 11 feet long.
 The yintercept means the cord is 10 feet long when no weight is applied.

Questions Eliciting Thinking If the company wanted to test the stretching behavior of a different type of bungee cord, how would they create a model of the relationship?
Will lengths calculated from this equation always match the lengths you might find through experimentation? 
Instructional Implications Ask the student to use the linear model to predict the length of a bungee cord holding 300 lbs.
Ask the student to explain the meaning of the slope in the context of the linear model.
Consider implementing other MFAS tasks for this standard (8.SP.1.3). 