If the graph of the line $y = ax + b$ passes through the points $(4,5)$ and $(8,17)$, what is $a - b$?
First let's find the slope $a$. To do that, we need to divide the difference in the $y$-values by the difference in the $x$-values. So that would be $\frac{17-5}{8-4}$. So the slope is $a=\frac{12}{4}=3$. Now we can use either of the points to find $b$. Let's use $(4,5)$. So we have $5=3(4)+b$. So $b=5-12=-7$. Therefore $a-b=3-(-7)=10$.

