In this problem F is the force acting on a spring with constant k. The force F creates a deformation, either by stetching or compressing the spring. Let x be this deformation.
The relation aming F, k and x is: F=k×x.
A. k=20N/m, F=100N.
x=F/k=100/20=5m
B. x=0.03m, k=300N/m
Fe=k×x=300×0.03=9N
