Rational expressions work like this: if you raise a number x to the power m/n, it means that you're considering the n-th root of x^m.
So, in this case, you are considering the second root (i.e. the square root) of 49/64 to the first power, i.e. 49/64 itself. In formula, you have

Since there is a minus sign in front of the whole expression, the answer is -7/8