Suppose $f$ is a function of the form $$f(x) = a^{bx}$$ for some constants $a$ and $b$. The doubling time for $f$ is $3$, i.e., $$f(x+3) = 2f(x)$$ for all $x$.
Then $f(x) =$ .
Hint: The choice of $a$ and $b$ are not unique, but the function is, and WeBWorK will accept any correct choice of $a$ and $b$. Just look for a doubling time of $3$.