No, use parentheses (as in the following):
... the answers are 2^x, 4^x, 2^(-2x), 4^(-2x). I got 2^(2x), which isn't one of the choices.
Suppose the actual answer is equivalent to 2^(2x), which is what you figured.
Can you convert that to one of the answer choices?
$\displaystyle 2^{2x} \ = \ 2^{2(x)} \ = \ (2^2)^x \ = \ ? $