The problem you are refered to is included in the book Mechanics 1 by Douglas Quadling for IG students of grade 11 ,page 142 ex 9.

For the part c just remember that W= TXh where T is the tension of the string.This W is transfered to kinetic energy (1/2 )mv^2

Substitute v^2 in the above expression and part c is resolved

Minoas