My niece told me that she needed some help... so, here I am begging for yours!

It's in spanish, so I'll translate as best as possible:

Let H be a subgroup of a certain group G and $\displaystyle a$ a fixed element of G. Prove that $\displaystyle K$ - the set of all the elements in the form $\displaystyle aha^{-1}$ with $\displaystyle h \in H$ is a subgroup of G.

$\displaystyle K$ ={$\displaystyle x \in G : x = aha^{-1}$, for some $\displaystyle h \in H$}