To calculate Hooke Law:

What you want to calculate

Spring Force Constant(k)
N/m

Distance from Equilibrium(x)
m

Spring Equilibrium Position(x_{0})
m

Force(F_{x})
N

Formula:
Force:
Spring Force Constant:
Distance from Equilibrium:
Spring Equilibrium Position:
where,
F
_{x} = Force,
k = Spring Force Constant,
x =Distance from Equilibrium,
x
_{0} = Spring Equilibrium Position.
In short Hooke's law can be explained as " F = kx" and to explain it further Hooke’s law states that force need to compress or extend a spring is directly proportional to the distance covered and constant factor i.e. stiffness is considered during this calculation.
Know more 
http://en.wikipedia.org/wiki/Hooke's_law