Answer:
[tex] {( \bold {\purple{ \boxed{ x + 7}}})}^{2} = \: \bold {\purple{ \boxed{ 9}}}[/tex]
Step-by-step explanation:
[tex] - 2 {x}^{2} - 28x = 80 \\ \\ - 2( {x}^{2} + 14x) = 80 \\ \\ {x}^{2} + 14x = - 40 \\ \\ {x}^{2} + 2.x. \bold{ \red{7 }}+ 49 = - 40 + 49 \\ (add \: square \: of \: 7 \: both \: sides)\\ \\ {x}^{2} + 14x + {7}^{2} = 9 \\ \\ {( \bold {\purple{ \boxed{ x + 7}}})}^{2} = \: \bold {\purple{ \boxed{ 9}}}[/tex]
