Hooke's Law states that the force needed to stretch a spring is directly proportional to the extension of the spring up to the limit of proportionality. 
(directly proportional = straight line through the origin)
F = kΔL
where ΔL is change in length.When you stretch a rubber band or spring the force needed to stretch the object is directly proportional to the extension provided the limit of proportionality is not exceeded. of the object. A force-extension graph will give a straight line through the origin. The gradient is the force per unit extension and is known as k the spring constant.Elastic Limit
If the spring is stretched beyond this limit it does not regain its initial length when the force is removed.Force-Extension Graph
(directly proportional = straight line through the origin)