Now, an ideal of a ring is a set which is closed under addition and under multiplication by and element of the ring. So, for all , and for all (although your ring is commutative so you do not need to show both and are in here).
So, you need to check these two things here.
Let and be in the ideal and let be an arbitrary element of the ring.
Does satisfy the condition? What about , once you have multiplied it out?