Answer:
1575 J
Step-by-step explanation:
The elastic potential energy of a string is given by:

where
k is the spring constant
 is the stretching/compression of the string with respect to its natural length
 is the stretching/compression of the string with respect to its natural length
For the spring in the problem, we have:
- Spring constant: 

- Stretching: 

Therefore, the elastic potential energy stored in the spring is
