If you were to solve following system by substitution what would be the best variable to solve for and from what equation? 2x+6y=9.

3x-12y=15

This is an odd question because it's a little subjective, but, for me, I would choose x from the second equation because you could simply add 12y to both sides, then divide by 3. It has the added benefit of not having any decimals/fractions, or negative values, which would be nice for substitution.