Linear Algebra - Proving a determinant

http://imgur.com/CyRy94H

I need help with part a. I know I'm suppose to factor out a (1-t) or a (1+t) somewhere along the way, and then do some row operations to get the determinant on the left, but i'm lost on how to get there. I've tried working from the left to the right, and the right to the left, but i'm not having much luck.