My teacher wants me to solve this by substitution:

$\displaystyle \int^7_0 \frac{x}{\sqrt{1+5x}}$

My work so far:

$\displaystyle u = 1+5x$

$\displaystyle du = 5$

$\displaystyle x = \frac{u-1}{5}$

$\displaystyle dx = \frac{1}{5}$

Not sure where to go from here...