How to solve this equation for y?
$$y^2+2yx-3x^2=0$$
I know how to factor the unknown y with the coefficients of the expression and it would be $(y+3)(y-1)$ but I don't remember how to work out x. Should work a kind of substitution method - indeed, I dealt already with this kind of task in the past but can't find it in my copybooks right now.
$y^2+2yx-3x^2=y^2-yx+3yx-3x^2=y(y-x)+3x(y-x)=(y+3x)(y-x)$