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

m=slope=$\frac{\Delta y}{\Delta x} = \frac{12 - 20}{36 - 20} = - \frac{1}{2}$
$y - y - 0 = m \left(x - {x}_{0}\right)$
$y - 20 = - \frac{1}{2} \left(x - 20\right)$ to find your equation $y = - \frac{1}{2} x + 30$.