Pls help- Piece wise functions

Suppose that $a$ and $b$ are positive real numbers, and let

\[f(x) =
\begin{cases}
\frac{a}{b}x & \text{ if }x\le-4, \\
abx^2 & \text{ if }x>-4.
\end{cases}
\]

If $f(-4)=-\frac{60}{13}$ and $f(4)=3120$, what is $a+b$?