Differential Equation problem:

A mass of 1 slug is suspended from a spring whose spring constant is9 lb/ft.The mass is initially released from a point1 footabove the equilibrium position with an upward velocity of

3

ft/s.

Find the times for which the mass is headingupwardat a velocity of3 ft/s.(Enter your answers as a comma-separated list. Letnrepresent an arbitrary integer.)

So for this problem I came up with the answer

t= (-pi/6) + (2npi/3) which is correct but according to the source I got it from is wrong.. What could be left to add here?

Thank you