An ideal spring of negligible mass is 11.00 cm long when nothing is attached to it. When you hang a 3.15- kg weight from it, you measure its length to be 12.40cm.

If you wanted to store 10.0 of potential energy in this spring, what would be its total length? Assume that it continues to obey Hooke's law.

Express your answer numerically. If there is more than one answer, enter each answer, separated by a comma.

m be the suspended mass,

g be the accelration due to gravity,

k be the spring constant,

x be the extension,

E be the energy to be stored in the spring.

mg = kx

k = mg / x

= 3.15 * 9.81 / (0.014)

= 2207

=221

E = kx^2 / 2

x^2 = 2E / k

x = sqrt(2 * 10 / 221)

= .301

=3.01cm

The total length is:

11.00 +/- 3.01

However this answer is not correct, what am I doing wrong? Any help would be appreciated