Find dy/dx

$\displaystyle y= sin^2(3x-2)$

I don't know how to work with $\displaystyle sin^2$.

I used the chain rule twice and got $\displaystyle 6 cos(3x-2)$ .

That, however, is not the correct answer.

It is $\displaystyle 3 sin (6x-4)$ .

Thanks!