### Home > AC > Chapter 7 > Lesson 7.3.1 > Problem 7-85

7-85.

The equation of a line is in the *y = mx + b* format, where *m* = slope (growth) and *b* = *y*-intercept (starting point).

Substitute the slope and the (*x*, *y*) point given in the problem into *y = mx + b*.

*y = mx + b**y* = 1.5*x* + *b*

32 = (1.5)(21) + *b*

Solve for *b*.

*b* = 0.5

*y* = 1.5*x* + 0.5

Don't forget to check your solution!

One way to find this is to substitute an *x* or *y* value of your choice into the equation you found in part (a).

Sam substitutes the value *x* = 4 into the equation.

*y* = 1.5 (4) + 0.5*y* = 6.5

The coordinates of the third point that Sam found are (4, 6.5).