Find the derivative of: 7sin2(9x4)\displaystyle {7}{{\sin}^{{2}}{\left(-{9}{x}^{{4}}\right)}}.

Hint: sin2(x)=[sin(x)]2\displaystyle {{\sin}^{{2}}{\left({x}\right)}}={\left[{\sin{{\left({x}\right)}}}\right]}^{{2}}...so use the chain rule (twice!).