Look at the picture.
The slope-point formula:
[tex]y-y_1=m(x-x_1)\\\\m=\dfrac{y_2-y_1}{x_2-x_1}[/tex]
We have the points (30, 120) and (40, 100).
Substitute:
[tex]m=\dfrac{100-120}{40-30}=\dfrac{-20}{10}=-2\\\\y-120=-2(x-30)\\\\y-120=(-2)(x)+(-2)(-30)\\\\y-120=-2x+60\qquad|+120\\\\\boxed{y=-2x+180}[/tex]
