If the graph of the line $y = ax + b$ passes through the points $(4,5)$ and $(8,17)$, what is $a - b$?
Slope = (8 - 4)/(17 - 5) = 4/12 = 1/3
Point-slope form: y - 5 = 1/3*(x - 4) = 1/3*x - 4/3
==> y = 1/3*x + 11/3
a - b = 1/3 - 11/3 = -10/3
