We know that

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}(sin(3x){)}^7(cos(3x){)}^{4}dx={\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}(sin(3x){)}^6(cos(3x){)}^{4}sin(3x)dx=\left[\begin{array}{c}u\\ du\end{array}\right]====$dots

Find the substitution $u.$

$u=-3sin(3x)$

$u=cos(3x)$

$u=sin(3x)$