linear transformation to matrix

1. Let T:R2 -> R2 be the linear transformation which stretches vectors by a factor of 2 along the x-axis and reflects vectors across the x-axis. Find the matrix associated with this transformation.

2. Let T:R2 -> R2 be the linear transformation which reflects vectors through the xy-plane. Find the matrix A associated with this transformation.

Just to be clear, I don't want an answer, but can you show me some basic steps on how to start for both problems? Thanks so much.