consider a right angle traingle of one of the angles theta (sorry formula editor not working)

perpendicular is 2

base is 3

hence hyp = root(13)

so cos(theta) = 3/ root (13)

sin theta = 2/root(13)

so y=  cos  inverse( 2cosx/root(13) + 3sinx/root(13))

 y=cos inverse(cos(x-theta)

y= x-theta

so dy/dx=1

=