Find the work done in streching the spring?

Question

If the natural length of helical spring is 10 inches and if it takes a force of 3 pounds to keep it extended 2 inches, find the work done in streching the spring from its natural length of 15 inches

Answer 1

k = 1.5 lb/in
W = (1/2)kx^2 = (1/2)(1.5)(5)^2
W = 18.75 in•lb