I am struggling with this.

A wombat of mass 22.0 kg starting at rest slides 13.0 m down a tree at a speed of 5.80 m/s just before reaching the bottom.
(a) What is the change in the gravitational potential energy of the wombat during the trip?
(b) What is the wombat’s kinetic energy just before landing with a loud thump at the bottom?
(c) What is the average force of friction that acts on the sliding wombat?

a) m g h

b) (1/2) m v^2

c) answer a - answer b = energy lost to friction

energy lost to friction = work done by friction (friction force on tree *distance down)
so
friction force = energy lost/13 meters