(a) Just take a small section, find its PE & integrate it
Mass of that small section dm = m/l * r * dθ
Height of that small section above center of sphere = r cosθ
PE of that small section dE = dm * r cosθ * g
=> E =∫ dE = mr2/ l * sinθ
Limit is from θ=0 to θ=l/r
=> E = mr2/ l * sin(l/r)
I think this was pretty much simple [1], try to solve b and c from urself.