\(\left(x+y\right)^2-4x^2\)
\(=\left(x+y\right)^2-\left(2x\right)^2\)
\(=\left(x+y-2x\right)\left(x+y+2x\right)\)
\(=\left(y-x\right)\left(y+3x\right)\)
\(\left(x+y\right)^2-4x^2\)
\(=\left(x+y\right)^2-\left(2x\right)^2\)
\(=\left(x+y-2x\right)\left(x+y+2x\right)\)
\(=\left(y-x\right)\left(3x+y\right)\)