19. (4 points) Let R be the region bounded by the graphs of y = 4x x2 and y = 3. Set up (but do not compute) the integral to nd the volume of the solid generated by revolving R about the line 3: = 1. 20. (4 points) Suppose that 2 J of work is needed to stretch a spring from its natural length of 10 cm to a length of 20 cm. How far beyond its natural length Will a force of 20 N keep the spring stretched