1. When factoring by grouping I am having problems understanding a particular step.

Expression:$\displaystyle 2x^4-6x-x^3y+3y$

$\displaystyle =(2x^4-6x)-(x^3y+3y)$

$\displaystyle =2x(x^3-3)+y(x3-3) $ Why are we able to change the signs towards the end of the expression? In the first step its a "-" sign and at the next step its a "+" sign.

$\displaystyle =(2x-y)(x^3-3)$ and in the final solution the sign changes again...

2. Reducing a fraction with -1

"Reducing a fraction or adding two fractions sometimes only requires that -1 be factored from one or more denominators"

Expression:$\displaystyle \frac{3}{y-x}+\frac{x}{x-y}=\frac{-3+x}{x-y}$

-The book lists the steps to solve this but they are rather confusing and they do not clearly explain, I would prefer a member on here run me through the above expression and provide the how/why when solving this problem.