Pls help me im confused

Which value of c would make he following expression completely factored? 8x+cy

Monica build a remote control toy airplane for science project to test the plane she launched it from the top of a building the plane travel a horizontal distance of 50 feet before landing on the ground a quadratic function which models the height of the plane in feet relative to the ground at a horizontal distance of x feet from the building is shown

Measures of the angle of r, 31 s (x+4) t (3x+9)