The question I am having difficulty with states that it requires 123 J of work to stretch a very light ideal spring from a length of 1.4m to a length of 2.9m. What is the value of the spring constant? My thinking was that the work required would be equal to the spring force so I set up 123=.5k(1.5)^2 getting 109.33 which is wrong. I dont just want the answer I would like to understand how this problem works. Thanks!