Question # 2 lol. I don't know why I'm having such a hard time getting this....

Determine the derivative of the following fuction:

g(x) = (cosx)^3

I started by using the power rule to come up with:

g '(x) = 3(cosx)^2

= 3(-sinx)^2

According to my book this is wrong though... it has:

g '(x) = (3(cosx)^2)(-sinx)

= -3cos^2xsinx

Which makes no sense to me. Thanks again.