A fractional exponent means raise the number to the power of the numerator and take the root indicated by the denominator. Here, you can either square -64 and take the cube root of the result, or you can take the cube root of -64 and square the result. The order doesn't matter except that the second way is computationally easier if you are doing it in your head. I did it in my head and got 16. Check it out on the calculator.