Proof on the divisibility of integers

[kolley]

Homework Statement

Let a,b be integers where a doesn't =0. Prove that if a divides b, and b divides a, then a=b or a=-b

The Attempt at a Solution

I started out with b=aj and a=bk, where j,k are integers. Don't quite know how to proceed

 

[Mark44]

Substitute the expression for a or b into the other equation. What do you get?