Answer:
[tex]{ \tt{3(x - y) - 7 {(y - x)}^{2} }} \\ \\\dashrightarrow \: { \tt{3(x - y) - 7(y - x)(y - x)}} \\ \\ \dashrightarrow \: { \tt{3x - 3y - 7 {y}^{2} + 14xy - 7 {x}^{2} }} \\ \\ \dashrightarrow \: { \tt{ - 7( {x}^{2} + {y}^{2}) + 14xy + 3x - 3y }} \\ \\ \dashrightarrow \: { \tt{ - 7 \{ {(x + y)}^{2} - 2xy \} + 14xy + 3x - 3y}} \\ \\ \dashrightarrow \: { \tt{7 {(x + y)}^{2} + 12xy + 3x - 3y }}[/tex]
