${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}(7x-2{)}^{8}dx$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}du$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{du}{u}$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}{u}^{n}du$

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}{e}^{u}du$

Identify $u.$

$u=$