Basically you need to argue the followings in order to show that is indeed an ideal of R.
1. Since , it follows immediately that .
Since , we see that . Thus .
Since and for some , we see that (Use a binomial exapansion ).
Argue that .
4. or for any .
Argue that or . Since R is a commutative ring (its ideals are two-sided), you need to show either of them.