Ta có:
\(A=x\left(x+2y\right)-2y\left(-x-2y\right)\)
\(\Leftrightarrow A=x\left(x+2y\right)+2y\left(x+2y\right)\)
\(\Leftrightarrow A=\left(x+2y\right)+\left(x+2y\right)\)
\(\Leftrightarrow A=\left(x+2y\right)^2\)
Thay \(x=7-2y\) vào biểu thức ta có:
\(A=\left(7-2y+2y\right)^2\)
\(\Leftrightarrow A=7^2\)
\(\Leftrightarrow A=49\)
Vậy \(A=x\left(x+2y\right)-2y\left(-x-2y\right)=49\)