Please see the attachments.
Use coriolis acceleration concepts and i j k vector coordinates to solve.
Coriolis acceleration = ac = 2 w x (dr/dt)
---> ac = 2wu
Angular velocity = w = 20*2pi rad/min = 20*2pi/60 = 2pi/3 rad/s
u = ds/dt
ds = displacemnt along AB = 20 in - 10 in = 10 in
dt = time lapse = d(theta)/w
d(theta) = rotation = 90-0 = 90 ...
This solution uses coriolis acceleration equations to find acceleration.