Because you have two variables and only one equation, you can only solve for approximate answers, and not complete, absolute answers.
You can simplify this equation to x = -4y + 12
y - 3 = -1/4(x + 3)
Distributive property.
y - 3 = -1/4x - 3/4
Multiply all terms by 4.
4y - 12 = -x
Multiply all terms by -1
-4y + 12 = x