The Solution:
We are required to find the force.

In this case,
F= force=30 N
a = acceleration = 3m/s
k = constant of proportionality = ?
We shall find the constant k by substituting the above values in the formula.

Dividing both sides by 10, we get

So,
The formula for finding Force is:

To find the Force when a = 3m/s, we shall substitute 3 for a in the formula.

Therefore, the correct answer is 9 N