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Pulley Mechanic Question

Could someone please help with this question, I understand the general idea and have solved all the other questions on this topic in the textbook, but am stuck on this one.

a 4500kg buggy is lifting a 300kg stone slab using a light inextensible rope which passes over a fixed, smooth pulley. Initially the slab is at rest on the ground bellow the pulley and the rope is taut. The buggy produces a driving force of 12000 N and is slowed by a constant resistive force of F N.

The slab reaches 2 ms^-1 after 6 seconds.
Find the magnitude of the resistance force F N.