Question

Solve the system of equations using the linear combination method. {4x−3y=12;7x−3y=3

Answer 1

4x−3y=12
7x−3y=3             Important:  you have two -3y terms.  Mult one eqn by -1 to 
                           obtain +3y, and then add the 2 equations together:

4x - 3y = 12
-7x+3y = -3
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-3x       = 9, so x = -3.  Subst. -3 for x in the first original equation to find y:

4(-3) - 3y = 12, or -3y = 24.  Then y = -8.

Solution is (-3, -8).  

It's important that you check this answer by subst. (-3, -8) back into either one of the original equations.