The mass of the rock is 15 kg if the weight is 147 N. If it is falling, we need more information.
The question requires some additional information to understand how the act of falling is significant. The downward force accelerating a falling 15 kg rock is 147 N.
But if the rock produces a force of 147 N when it hits the ground, we would need to know the distance it dropped and how quickly it comes to a stop. That would be given by the equality of the 2 ways to calculate impulse. Change in momentum and Forcetime
are both ways to calculate impulse.
Impulse = mass(final velocity - initial velocity) = average force*time
I hope this helps,