Question

A spring stores 5j of energy when stretched by 25 cm.

Answer 1

w = (1/2) k x²

x = 0.25 m 
w = 5j
k = 2 w / x² = (2 x 5)/0.25 x 0.25  =  10 / 0.0625 =  160 N/m 

hope , it helps