[lukevanoort]

You can also factor it out into two quantities. Eg. x^2+3x+2=0 factors out into (x+1)*(x+2)=0 since one of those quantities must equal zero and it could be either one you set both equal to zero, so it would be x+1=0 and x+2=0. These solve out to x=-1 ans x=-2. (-1)^2+3(-1)+2 goes to 1+-3+2 which proves the x=-1 answer. Then we do the other one, (-2)^2+3(-2)+2 goes to 4+-6+2 which equals zero too. So now we are 100% sure that it is x=-2 and x=-1.