3x + 12 = 24, solve for x.

Firstly, we need to make 'x' the subject of the equation. We do this by 'moving' everything that isn't an x over to the other side of the equation. When we do this, the main thing to remember is that whatever we do to one side of the equation we must also do to the other side. Think of the equals sign like a balanced weighing scale, if you start removing weights from one side of the scale then they will no longer be balanced and hence you need to remove the same weight from the other side in order for them to remain balanced. This is the same with a mathematical equation! What ever you do to one side of the equals sign you need to do it to the other side in order for it to remain balanced.
Lets first think about the plus 12 on the left hand side. In order to get rid of this, so that we can get the x by its self on the left hand side, we must minus 12 from both sides. Now the equation becomes 3x = 12. Then in order to get x to be the subject of the equation, we need to do something about the 3 in front of the x. Currently we have '3x' which is 3 times x. The opposite of times is divide and hence that is how we can remove the 3 to get the x by its self. So lets divide by 3 and then we get, x = 4. Problem solved!