Consider the problem of computing an exponential sum over a certain function $g(x)=f(x)+h(x)$, that is computing
$$\sum_{x}g(x)=\sum_{x}f(x)+\sum_{x}h(x)$$
now if we know that $\sum_{x}f(x)$ and $\sum_{x}h(x)$ are two NP-Hard problems, what can we say about the hardness of $\sum_{x}g(x)$?