1. If you stretch a spring the necessary force is not constant but is proportional to the distance you have stretched the spring:
If F is the force, d is the distance of stretching then:
. This quotient is constant (in certain boarders: You can't stretch a spring of 10 cm to a length of 1 km). In your case:
2. Since the force is not constant your formula to calculate the work is not valid. The work done to stretch a spring can be calculated by:
. Plug in the values for k and d.