Answer:
C. -12 ab
Explanation:
The restoring force on a spring is given by Hooke's law:
where
k is the spring constant
x is the stretched (or compressed) displacement of the spring
In this problem we have:
k = 4a
x = 3b
Substituting into the equation, we find:
And the negative sign means that the direction of the force (negative) is opposite to the direction of the displacement (positive).