integrate the problem
integral(4x/sqrt(8x^2-1), dx, 2, 1)
$\displaystyle \int^{2}_{1} \frac{4x}{\sqrt{8x^2-1}} dx$
Factor out your 4.
$\displaystyle 4 \int^{2}_{1} \frac{x}{\sqrt{8x^2-1}} dx$
Do a substitution with:
$\displaystyle u = 8x^2-1$
$\displaystyle du = 16x dx$
*Change your limits by plugging in your original limits into the equation for u
$\displaystyle \frac{1}{4} \int^{31}_{7} \frac{1}{u} du$
$\displaystyle = [\frac{\sqrt{u}}{2}]^{31}_{7}$
Then evaluate!