f(b) means evaluate f(x) at the value b. In this case f(3) means find f(x) when x=3.
Since you have the constant $a$ you can find it's value from f(3) [64 is a cube number so $a$ is a whole number] as an intermediate step. You won't need a calculator for this one but as a hint $64 = 4^3$
Solve $f(3)$ in terms of $a$ by substituting $x=3$ in $f(x)=a^x$
Work out the value of $a$ from the above since you're told that f(3) = 64
Use that value of $a$ to find $f(2) $ by substituting $x=2$