I presume you are talking about G-sets.

The notation I will use is, for the group.

And for the non-empty set.

The binary operation will simply be juxtaposition of two elements,

The standard way to show,

It to show,

Now,

Let,

Then,

Because,

Thus,

implies

Similarly,

Thus,

implies .

Thus,

(The facts that the sets are finites and groups are finite was not necessary).