I need to figure out what my A is and what my B is.

The question is:

Prove by contrapositive that 1+sqrt(2) is irrational. So how do I define my A and B.

all I can think of is A: consider two numbers 1 and sqrt(2)

B: their sum is irrational.

I can prove it but I feel like my A and B are completely wrong. Thanks so much