Let us consider the electric charge is accelerated through a distance s under a potential difference V.
hence electric force acted on the charge is F = qE = q(V/s).
acceleration acquired by the charge is (F/m) = (q/m)(V/s).
if the charge started with zero velocity, after travelling a distance s and entering into the magnetic field with speed v = 2 (q/m)(V/s)×s = (2qV)/m
If radius of circular path travelled by the charge is d/2 (given), then we can write