TLDR: I want to solve function $(-4x^2-6x+4)/(x^2+1)^2$ for $0$. How can I get the polynomial out of the denominator so I can apply the quadratic formula?
Long form: I'm trying to find the horizontal tangents of the function $f(x)=(4x+3)/(x^2+1)$, I'm pretty sure I got the first derivative correct and I want to set the result of the derivative to $0$ so I can find all the line formulae where the slope is $0$.