David G. answered 09/20/13
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4x + 12 = 3x - 6 + 3x
4x + 12 = 6x - 6 (add the 3x's together)
12 = 2x - 6 (subtract 4x from both sides of equation)
18 = 2x (add 6 to both sides of equation)
9 = x (divide each side of equation by 2)