Solve x+3y=12

There is no single "solution" to this equation. You can see that easily:

9 + 3 * 1 = 12

6 + 3 * 2 = 12

so this does not specify a unique x or y, but for any given x, there can be only one y.

In fact, it specifies an infinity of (x, y) pairs, that, if you graph them, will make a straight line.