finding the complex amplitude

[skywo1f]

1. The problem statement, all variables and given/known data
i gotta find the complex amplitude
i know that w = 50Hz
displacement was 30mm at time 0
and -14mm at time 12ms



2. Relevant equations

(displacement) = Acos(phase constant)cos(wt) - Asin(phase constant)sin(wt)

3. The attempt at a solution
so ive tried plugging things in to the expanded wave equation
and just plugging things in for t = 0
which gives me the correct answer Acos(phase constant) = .03 at time 0
but when i try the second time to get the sin phase constant, it gives me .068
but my answer is supposed to be
.03 +.0175 i
what am i doing wrong?
which equation could i use?
any help would be appreciated

[skywo1f]

nvm figured it out
i was using 50 hz as my w
but really its my v
and i had to convert it with v = w/2pi