Great, let's write the equations with our x's, y's and #' all lined up and ADD the two equations together:
2x + y = 9
(+) x - y = 3
3x + 0 = 12
**NOTE that y + (-y) = 0, because those cancel out.**
So now we're just left with 3x = 12. If we divide both sides of that equation by 3, we get x = 4.
(BTW, often, the question will also ask you to solve for the y-value. To do that, substitute the answer that we just got for x into either equation to solve for y. One way to check your answer actually, is that you should get the same answer for y from both equations.)
