I am having a terrible time understanding this one:

Prove that for all x E r, and y E R, if x is rational and y is irrational, then x + y is irrational. use proof by contradiction known as contrapositive.

OK I understand that contrapositive (~p --> ~q) so

Proof: if x is not rational then y is not irrational

Now I have no idea what to do next?

Please help, I have a test coming up in two days and I need to grasp this stuff.