Is this the one?

Take x + 7 = 12. To solve is to find the number x stands for. You do not have to solve it yet. Just test a number in it. Try 4.

Try it, then compare the sides

Put 4 in place of x. The left side is 11. The right side is 12. They differ, so 4 is not the answer.

Put 5 in. Both sides are 12. They match, so 5 is the answer.

A number that makes an equation true is its solution.

Rule: Put the number in. Work out each side on its own. It is a solution when both sides match.