Question

Factor using ac-method,  15y+6y²-21= ________

Answer 1

$15y+6y^2-21=\\ 6y^2+15y-21=\\ 3(2y^2+5y-7)=\\ 3(2y^2-2y+7y-7)=\\ 3(2y(y-1)+7(y-1))=\\ 3(2y+7)(y-1)$