Question de73f17a

Function $f$ is defined by $f\left(x\right)=-a^x+b$, where $a$ and $b$ are constants. In the xy-plane, the graph of $y=f\left(x\right)-15$ has a y-intercept at $\left(0,-\frac{99}{7}\right)$. The product of $a$ and $b$ is $\frac{65}{7}$. What is the value of $a$?