2/5x - x/4 = 4
$\displaystyle \frac{2}{5x} - \frac{x}{4} = 4$
Find a common denominator. Easy way to do that is multiply all denominators with each other: 5x*4 = 20x. Now, multiply the entire equality by 20x (same as multiplying each term by 20x).
$\displaystyle \frac{40x}{5x}- \frac{20x^2}{4} = 80x$
Simplify it...
$\displaystyle 8-5x^2=80x$
$\displaystyle 5x^2+80x-8$
Quadratic formula will solve it...
$\displaystyle x = \frac{-80 \pm \sqrt{80^2 - 4(-8)(5)}}{10}$
$\displaystyle x = \frac{-80 \pm -80.994}{10}$
$\displaystyle x = -8 \pm - 8.0994$
$\displaystyle x = 0.0994, -16.0994$