General

Help with a differential equation problem?

The problem is:

Find the steady periodic solution to the differential equation

x″+3x′+16x=4sin(2t)

in the form

xsp(t)=Ccos(ωt-α),

with C > 0 and 0≤α<2π.

I have C and ω as 2sqrt(5)/15 and 2 respectively and my A and B values from solving the particular solution are A=(4/15) and B=(-2/15). So then I use arctan(B/A) to find α and I add 2π to it because it's in the fourth quadrant, but the answer is wrong. Help??