SAT Lines, angles, and triangles Question

Note: Figure not drawn to scale. In the given figure, NP is the diameter of semicircle 0. Angles Q, R, and S are right angles, and point P is the midpoint of NQ. If PQ=52 feet and NS=18 feet, the area of the figure can be expressed as aπ+b square feet, where a and b are integers. What is the value of $a-b$?