A spring of spring constant 5x103 N/m  is stretched initially by 5 cm from the unscratched position. Then the work required to stretch it further by another 5 cm is

for a spring .U =12kx12where x1 =5 cmWhen the spring is further stretched by 5 cm, thenU′=12kx22Now W=U′−U=12kx22−x12=125×10310×10−22−5×10−22 =18.75Nm