We don't not know what would x really equal. So when solving this, it would look like the problem would be not quite finished, but it would be, so we would be subtracting this, but keeping x as it's self.
Let's talk x out of the whole problem.
(4-12) = 8. This would be the answer so far.
Then we do, (8-18) = 10.
Now we can bring x back in the problem.
Your answer would be (10x).
