Answer:
 250J
Step-by-step explanation:
Given parameters: 
Spring constant = 500N/m 
Extension = 1m 
Unknown: 
Elastic potential energy = ? 
Solution: 
The elastic potential energy of a body is the energy stored within an elastic string. 
 EPE = 
 k e² 
k is the spring constant
e is the extension 
 Now; 
 EPE = 
 x 500 x 1² = 250J