wel its actually (-x)+(-x)=-2x
it would not be an exponent because you are not multiplying.
if you substitute x for any number, as long as both of them are, you will see.
also, if you think about it, there is the 2 x's, which is where the 2's come from. and there are negative signs which makes them the same, so you can combine them. so it is saying there are 2 negative x's