Answer: 5^15
The rule used is (a^b)^c = a^(b*c)
If you raise some exponential expression to another exponent, then you multiply the exponents. You keep the base the same.
In this case, a = 5, b = 3, c = 5.
Using a calculator,
5^15 = 30,517,578,125
But it's likely your teacher will want you to keep the answer in exponential form since it's easier to work with.