This is from simple math knowledge of lines, segments and points. From the given data, pr and rs are line segments in the line ps. Thus, we add segments pr and rs together to solve for the line ps.
pr + rs = ps
4x - 2 + 3x - 5
(Add like terms) = 7x -7
(Further simplify) = 7(x-1)
Therefore, the final answer or expression of ps = 7(x-1).