Suppose that a and b are positive real numbers, and let
\(f(x) = \begin{cases} \frac{a}{b}x & \text{ if }x\le-4, \\ abx & \text{ if }x>-4. \end{cases}\)
If f(-4) = -60/13 and f(4) = 3120, what is a + b?
a + b = 12.
