Anstract Algebra proof - cosets

Homework Statement

Let H be a subgroup of G such that g^-1hg is an element of H for all g in G and all h in H. Show that every left coset gH is the same as the right coset Hg.

The Attempt at a Solution

need to show gh1=h2g
I know I need to show this, but am unsure on how to.