Let these variables represent the following
F_x=Fx= the force applied to the spring
k=k= the spring force constant
x=x= the distance from equilibrium
x_0=x0= the spring equilibrium position
The equation to determine the spring constant, kk is
k=(F_x)/(x-x_0)k=Fxx−x0
We are given the total measure of elongation is 44 cmcm, which is the difference between xx and x_0x0. That is,
x-x_0=4x−x0=4 cm=0.04cm=0.04 mm
We are also given the mass of the body, 1.51.5 kgkg, and assuming this measurement is happening on Earth, we have the acceleration due to gravity 9.89.8 ms""^(-2)−2.
F_x=9.8Fx=9.8 (ms^-2)xx1.5(ms−2)×1.5 kg=14.7kg=14.7 NN
Therefore, the final solution is
k=(F_x)/(x-x_0)=(14.7" N")/(0.04" m")=367.5k=Fxx−x0=14.7 N0.04 m=367.5 Nm^-1Nm−1