\[2x \cdot (2x + 3y) - (x + y)^{2} =\]
\[= 4x^{2} + 6xy - \left( x^{2} + 2xy + y^{2} \right) =\]
\[= 4x^{2} + 6xy - x^{2} - 2xy - y^{2} =\]
\[= 3x^{2} + 4xy - y²\]