How to find the invertible elements of $\Bbb{Q}[X]$ mod $X^2$ 
I am looking for some hints for finding all the invertible elements of $\Bbb{Q}[X]$ mod $X^2$.

Thank you very much in advance.

The ring $\mathbb{Q}[X]/(X^2)$ is local, that is, it has a unique maximal ideal.
An element of the maximal ideal is not invertible (prove it). What about the elements not in the maximal ideal?
