How would I solve this? (part one)

3) Circular motion of charges: As shown in the figure, a small particle of charge q = - 7.0 x 10-6 C and mass m = 3.1 x 10-12 kg has velocity vo = 9.4 x 105 m/s as it enters a region of uniform magnetic field. The particle is observed to travel in the semicircular path shown, with radius R = 11