How do you solve the system 3x - 4y = -1 and 9x + 12y = 15?

1 Answer
May 23, 2015

Isolate one variable in one equation and then substitute it in the other equation.

Let's, for example, isolate y in the second equation:

y=15/12-(9x)/12=5/4-(3x)/4=(5-3x)/4

Substituting it in the other equation (that is, the first one):

3x-cancel4(5-3x)/cancel4=-1
3x-5+3x=-1
6x=4
x=2/3

If x=2/3 and y=(5-3x)/4, then,

y=(5-cancel3(2/cancel3))/4=(5-2)/4=3/4