The perfect square
Let's highlight the perfect square of the square three-member
$$- y^{2} + \left(- x^{2} + 4 x y\right)$$
Let us write down the identical expression
$$- y^{2} + \left(- x^{2} + 4 x y\right) = 3 y^{2} + \left(- x^{2} + 4 x y - 4 y^{2}\right)$$
or
$$- y^{2} + \left(- x^{2} + 4 x y\right) = 3 y^{2} - \left(x - 2 y\right)^{2}$$
General simplification
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$$- x^{2} + 4 x y - y^{2}$$
/ / ___\\ / / ___\\
\x - y*\2 - \/ 3 //*\x - y*\2 + \/ 3 //
$$\left(x - y \left(2 - \sqrt{3}\right)\right) \left(x - y \left(\sqrt{3} + 2\right)\right)$$
(x - y*(2 - sqrt(3)))*(x - y*(2 + sqrt(3)))
Rational denominator
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$$- x^{2} + 4 x y - y^{2}$$
Combining rational expressions
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$$x \left(- x + 4 y\right) - y^{2}$$
Assemble expression
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$$- x^{2} + 4 x y - y^{2}$$
$$- x^{2} + 4 x y - y^{2}$$
$$- x^{2} + 4 x y - y^{2}$$
$$- x^{2} + 4 x y - y^{2}$$
$$- x^{2} + 4 x y - y^{2}$$