[tex]\sf \: {x}^{2} + 4xy - 21 {y}^{2} [/tex]
[tex]\sf \: Rewrite \: the \: equation⟹( {x}^{2} - 3xy) + (7xy - 21y {}^{2} )[/tex]
[tex]\sf \: 1)( {x}^{2} - 3xy) = x(x - 3y)[/tex]
[tex]\sf2) \: (7xy - 21 {y}^{2} ) = 7y(x - 3y)[/tex]
[tex]\sf \: Now \: the \: equation \: becomes \: ⟹ \\ \sf \: x(x - 3y) + 7y(x - 3y)[/tex]
[tex]\sf \: Factor \: out \: the \: common \: term \: (x - 3y) \\ \sf =( x - 3y) + (x + 7y) \\ [/tex]
Answer ⟶ [tex]\boxed{\sf {B) (x-3y)(x+7y)}}[/tex]
