What am I doing wrong with finding the complex amplitude?

[skywo1f]

Homework Statement

i got to find the complex amplitude
i know that w = 50Hz
displacement was 30mm at time 0
and -14mm at time 12ms

Homework Equations

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

The Attempt at a Solution

so I've tried plugging things into 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