Okay I'll tell you the solution.

So your given informations are

Amplitude A = 5 cm

Periodic Time T = 6 sec

To find phase at a distance of 2.5 cm from mean position.

Use the Formula y=A sin wt

2.5 = 5 sin 2pi/6 t

On solving you get,

Pi/6 = 2pi t/6

ie time = 1/2 seconds

Phase as given is 2pi t/6

ie 2pi/6 * 1/2 = pi/6

So the phase and the answer is pi/6

Thank you.