A particle (m=3*10^-6 g, q=5*10^-6 C) moves in a uniform magnetic field given by (60j) mT. At t=0 the velocity of the particle is equal to (30j - 40k)m/s. The subsequent path of the particle is?

I know that the answer is: helical with a 40-cm radius. But i don't know the solution to get to this answer. I really need help. Thanks for anything!