Question

How do you derive formulas, using y=mx+b, with the points (20,20) and (36,12)?

Answer 1

Remember that:
m=slope=`(Deltay)/(Deltax)=(12-20)/(36-20)=-1/2`
use it together with. say, the coordinates of your first point in:
`y-y-0=m(x-x_0)`
`y-20=-1/2(x-20)` to find your equation `y=-1/2x+30`.