To solve this problem, all of the data needs to be put on one side of the equation, and the sides needs to be subtracted from the perimeter. Therefore the beginning equation would be P - sides known = side unknown. (9x + 2) - ((2x -1) + (x +5) = side 3. (9x + 2) - (3x + 4) = side 3. 9x + 2 - 3x - 4 = side 3. So side 3 is (6x -2).