Answer: (r^6)/(s^3)
Reason:
When we cube the (r^2) in the numerator, we're multiplying it by itself 3 times
(r^2)^3 = (r^2)*(r^2)*(r^2) = r^(2+2+2) = r^6
or a shortcut is to say
(r^2)^3 = r^(2*3) = r^6
That explains how we get r^6 in the numerator of the final answer.
The s^3 in the denominator is simply the result of cubing the 's' in the original expression.