You're doing it wrong.
How to do this problem:
U(initial) {which is gravitational potential energy} = mgh, where h is 5m, m is 3kg, g is 9.81m/s^2.
U(final) {which is elastic potential energy} = 1/2kx^2, where k is 100.
WE WANT X.
W=delta E
Conservation of energy in sys mass, plane, earth...