The equation for force of a spring is F = -k(delta x)
In this problem, the force is 6 N, and the displacement (delta x) is 0.2 m ----> 6 N = -k * 0.2 m
Now just use algebra to isolate k on one side of the equation.
-k = 6 N/0.2 m = 30 N/m
k = -30 N/m
