Function $f$ is defined by $f(x)=-a^x+b$, where $a$ and $b$ are constants. In the $xy$-plane, the graph of $y=f(x)-8$ has a y-intercept at $(0,-\frac{33}{5})$. The product of $a$ and $b$ is $\frac{42}{5}$. What is the value of $b$?