Use the properties of exponents to simplify

(r^-3s^1/3)^6/(r^7s^3/2)

(r^-3s^1/3)^6 / (r^7 s^3/2)

r^-18s^2 /(r^7 s^3/2)
r^-11 s^1/2
or
s^1/2 / r^11