2x^2 - x - 1
ok what i like to do is this...
look at the expression you know that there are 2 x's so you pull these out first keeping in mind that there is a coefficient
(2x )(x )
now think what 2 numbers gives me "-1" you think "1" and "-1" (remember that you might have a few different factors)
if you plug those in... (remember that where you place these numbers does matter)
now check using FOIL !!
[latex]2x^2 - x - 1 =\\ 2x^2-2x+x-1=\\ 2x(x-1)+1(x-1)=\\ (2x+1)(x-1)[/latex]