Question is attached. It's driving me nuts, I can't understand what is going on at all. After being released from C, the particle starts moving with SHM, with the centre of oscillation at the midpoint of AB. It says the amplitude is 0.4m, but I would have thought that C would be the 'extreme' point of the oscillation, i.e. the particle hits C and turns back around, making the amplitude 0.5m.
Also, a general question - I have no idea how to work out where the centre of oscillation is. When it's just one spring, I know it's where the spring is at its natural length, but when a particle is attached to two springs which are stretched out, I'm baffled. Does the point where each spring is at its natural length have any significance at all?