Hook's law states that the relationship between the force F applied to a spring and the elongation/compression 
of the spring (with respect to its equilibrium position) is
where k is the spring constant, and the negative sign simply means that the direction of the force is opposite to the displacement.
In our problem, the spring is stretched by
And so we can use Hook's law to find the spring constant (we can ignore the sign and consider only the magnitude of the force, 16 N):
where k is the spring constant, and the negative sign simply means that the direction of the force is opposite to the displacement.
In our problem, the spring is stretched by
And so we can use Hook's law to find the spring constant (we can ignore the sign and consider only the magnitude of the force, 16 N):