If you were to solve the following system by substitution, what would be the best variable to solve for and from what equation? 3x+6y=9 2x-10y=13 a) x, in the second equation b) y, in the first equation c) y, in the second equation d) x, in the first equation

3x+6y=9 would be the best choice, since all of its terms are divisible by 3:

x+2y = 3, so x=3-2y

Subbing this result into 2x-10y=13 turns out 2(3-2y) - 10y = 13, or

6 - 4y - 10y = 13

6 - 14y = 13

-14y = 7, and thus y = -1/2.     Then x = 3-2y (as found earlier), and
                                                         x = 3-2(-1/2)  =  3+1 = 4

The solution is (4, -1/2).