Two forces, of magnitude F1=50.0 N and F2=22.0 N, act in opposite directions ona block which sits atop a horizontal frictionless surface as shown. Initially, the center of the block is at position xi=-4.00 cm. At some later time, the block has moved to the right, and its center is at position xf=+6.00 cm. a) What is the work done by force F2 as the block moves from x=xi to x=xf? b) What is the change in the block's kinetic energy as it moves from x=xi to x=xf?