How can one go from x = ( exp[(-g + iw)t] ) (a*sin[wt] + b*cos[wt])

How can one go from

x = ( exp[(-g + iw)t] ) (a*sin[wt] + b*cos[wt])

to

x = [A*exp(-gt)]*cos(wt + r),

where A = sqrt(a**2 + b**2) and r = -arctan(b/a) ? Note that "i" is the imaginary constant.