Hi I've been given the following exercise:

Let FGH be the composite of three isometries. Assume that there inverses exist. Prove that (FGH)^-1 exists, and express this inverse in terms of the inverses of the individual isometries.

I would greatly appreciate any hint about where to start and any general advice about attempting to solve problems of this sort. I need to rewire my brain, and I can't quite get it yet.

Thanks,

Ultros