Initial Value Problem - Differential Equation!!!

Hi, For the following problem I am having hard time trying to find the phase angle (alpha).

How to find the phase angle,

isnt that alpha = arctan(B/A) if A,B>0 ?

Problem:

Solve the initial value problem

(where primes indicate derivatives with respect to *t*). Express your solution as the sum of two oscillations,

x(*t*) = (49/27)*cos(6t - https://instruct.math.lsa.umich.edu/...6116b71031.png) + (5/27)cos(3t)

where *https://instruct.math.lsa.umich.edu/...31d28e5821.png = *https://instruct.math.lsa.umich.edu/...6116b71031.png

*(Recall that https://instruct.math.lsa.umich.edu/...31d28e5821.png must be between 0 and 2https://instruct.math.lsa.umich.edu/...06b3a9b1d1.png.)*

**But the phase angle is not right.**

Please help,

Many Thanks.