This Instructional Activity is a part of the Lesson 19, Unit 4, Grade 8. Now that all of the necessary tools have been developed, we are ready to show that the graph of a linear equation in two variables in the form y = mx + b, where b is always a constant, is a non-vertical line. To prove that the graph of a linear equation is a line, we need to show the following: Any point on the graph of y = mx + b is a point on a line l, and any point on the line is a point on the graph of y = mx + b.