A subgroup is a set of elements from G which form a group under the operation of G.

So, a subgroup will always contain the identity, in this case denoted 1. If it is non-trivial it will contain other elements too, for instance -1.

So, take the set

. Firstly, you should note that you have associativity as you inherit this from the group itself. You also have the identity element, this is just 1.

Can you find an inverse for -1? Is this in the set?

What about closure? If you multiply two elements from this set are we still in the set? The only non-trivial product is

, but this is still easy...