Взять производную....
(x²-4)`=2x (cosx/2+2sin3x)`=-1/2*sinx/2+6cos3x (3/(x+4))`=-3/(x+4)² (3x/(x+4))`=(3(x+4)-3x*1)/(x+4)²=(3x+12-3x)/(x+4)²=12/(x+4)²