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

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Answer 1 · 2015-05-23T01:25:01

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#