A SHM problem

A block of 5kg connected to the free end of a spring is hanging from the ceiling and is in SHM. At t=0, the block is travelling upwards and its displacement is 15cm from the equilibrium. The block reaches its maximum displacement 0.3s later. What is the general equation of the displacement of the oscillating block_

y=A cos (ωt+ø)

I use t=0, y=15cm and t=0.3, y=A to get 2 equations:

15=A cosø
0.3ω+ø=0

However there are 3 unknowns in these 2 equations, and I just do not know how to get the 3rd equation from the given quantities. Is this question miss something? Should it tell me the value of force constant k or sth. else? Please help to solve, thanks a lot!

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