W = {a + t^2, aER}; V = P2

So bascially what I need to do is to determine if a + t^2 is a subspace of P2 but I'm not too sure on how to do it. In lectures we learnt subspaces using vectors where we had to determine using the zero vector then check if it is closed under addition and Scalar multiplication but I don't know how to do it when it relates to a polynomial so I hope someone can just give me a few pointers