Since the exponent (-3) is negative, flip the expression to: 1/(5n^4)^3.
Notice how the negative exponent becomes positive as you flip it.
Now evaluate the powers in the denominator: 1/((5^3)(n^4)^3
I separated the constant (5) from the variable (n) to show you how the powers are evaluated.
1/(5x5x5)(nxnxnxn)(nxnxnxn)(nxnxnxn) —-> the power four means that there are 4 multiples of n in the parentheses. The power 3 corresponds to how many groups.