Derivative of $f(x)=-6\sin^4 x$ 
$f(x)=-6\sin^4x$  

$f(x)=-6\sin x^4$  
$f'(x)=-6\cos x^4(4x^3)=-24x^3\cos x^4$  
What am I doing wrong? Please show the steps.

$f(x)=-6\sin^4 x=-6(\sin x)^4$
By Chain Rule,
$f'(x)=4\cdot-6(\sin x)^3(\sin x)'=-24(\sin x)^3\cos x=-24\sin^3x\cos x$
