Advanced GCSE Indices

Question

At school, we've been asked to simplify the following: $$\left(\frac{S^{9/10}}{S^{3/5}}\right)^{-4}$$ I've given it a go but haven't been taught this yet. Could someone please explain the process which one would use?

Answer 1

$$\left(\frac{S^a}{S^b}\right)^c=(S^{a-b})^c=S^{ac-bc}$$