Work Power Energy question bank (Physics)

The spring is initially in an equilibrium condition. Initially it is stretched by length $l$ and the work done is $U_1$. Again, it is stretched by length $\dfrac{7l}{2}$, when the work done is $U_2$, the value of $U_2$ is

When a long spring is stretched by $1\ \text{cm}$, the potential energy stored in the spring is $U_1$. If it is stretched by $4\ \text{cm}$, the potential energy stored in it is $U_2$. The difference between the energy is