The spring formula:
Force = - (spring constant) · (distance stretched or squashed)
6 N = - (spring constant) · (33 cm)
= - (0.33 m) · (spring constant)
Spring constant = - (6 N) / (0.33 m)
= - 18.18 N/meter .
The negative signs means that the spring always wants to go in the
opposite direction from where you forced it, and return to its relaxed
length. If you stretch it, it tries to get shorter, and if you compress it,
it tries to get longer.