It's very simple. Watch
First we subtract 18 from both sides giving us:
But, we're not done. We must find a solution for r, not -r. To find r, we multiply both sides of the equation by -1.
To check this we start with our original equation:
Substitute 57 in for r to get:
Notice, the negative sign was already part of the equation. I just replaced r with 57. Now the check is fine.