I see an x and a factor of (4x - 1)^{-1} in both terms.
It's no different from an expression with positive exponents (if that makes you feel any better, first do it with +2 and +1)
Without negative exponents, your expression can be written as
[tex]\frac{3x}{(4x - 1)^2} - \frac{6x^2}{(4x - 1)}[/tex]
If you get a common denominator, it should be more obvious what you can factor out of each rational expression.