Find equation for trig word problem

the word problem is:

A mass is supported by a spring so that it is at rest 0.5 m above a tabletop. The mass is pulled down 0.4 m and released at time t = 0, creating a periodic up and down motion that can be modelled using a trigonometric function. It takes 1.2 seconds to return to the lowest position each time.

so i know it will be a sin(x) graph, and i know it starts 0.5m above ground level, therefore the vertical shift is 0.5m - that will give us:y = a sin (x) + 0.5

the mass will be pulled down 0.4, and will rise 0.4 above where it was stationary (so 0.4m + 0.5m = 0.9m is the maximum). I can determine the amplitude: (0.9 - 0.8)/2 = 0.4, so that will give us:

y = 0.4 sin (x) + 0.5

the only two things i'm not sure about is the phase shift and the horizontal stretch.