What is the solution of the system?{−3x+9y=364x+12y=24Enter your answer in the boxes.

What you must do for this case is to solve the system of two equations with two unknowns.
 To find the values of x and y.
 We have then:
 -3x + 9y = 36
 4x + 12y = 24
 Resolving:
 Step 1: 
 (1/3) * (- 3x + 9y) = (36) * (1/3)
 (1/4) * (4x + 12y) = 24 * (1/4)
 -x + 3y = 12
 x + 3y = 6
 Step 2:
 6y = 18
 y = 18/6
 y = 3
 Step 3:
 -x + 3y = 12
 -12 + 3y = x
 -12 + 3 (3) = x
 x = -3
 answer:
 x = -3
 y = 3