Student is tuning his guitar. He stretches the string by 10 mm so it produces the

Student is tuning his guitar. He stretches the string by 10 mm so it produces the right note when played. The string has a spring constant of 20 N/m.
Calculate the energy stored in the elastic potential energy store of the string as it is stretched.
You can assume that the limit of proportionality was not exceeded whilst it was being stretched.