Answer :

[tex]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) [/tex]

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