As per forum policy, we require that you show some attempt at the solution before we assist you. However, I'll start you off with a hint--- Try looking at the total torque on some charge and current distribution and follow a similar method as Jackson's derivation of conservation of linear momentum (eq. 6.121)
P.S. Please don't create multiple threads for the same question; that is also against forum rules. Since ths is essentially a homework problem, this is where this question belongs.