Simplify :

\(\sin(3t+\pi)+2\cos(3t)\\ =\sin(3t)\cos(\pi)+\cos(3t)\sin(\pi)+2\cos(3t)\\ =\sin(3t)\cdot(-1)+0+2\cos(3t)\\ =2\cos(3t)-\sin(3t)\)

= 2cos(3t) - sin(3t)