Conservation of energy is used in 2b(iii)
Method A: 1. All changes (KE + PE ) = W (on the system) or
2. All changes (KE + PE + W(heat) )=0
Choose either one or two which ever you can understand better.
The first one means if you apply force the work is positive Fd and there will be increase in (KE + PE)
Use the second one to solve (iii)
It will translate to mean that increase in KE will accompanied by decrease in PE and increase in heat as friction exist.Mathematically the increase in KE = change in PE -work done against friction taking the magnitudes only.