The perfect square
Let's highlight the perfect square of the square three-member
$$- 4 p^{2} + \left(p y - y^{2}\right)$$
Let us write down the identical expression
$$- 4 p^{2} + \left(p y - y^{2}\right) = - \frac{15 y^{2}}{16} + \left(- 4 p^{2} + p y - \frac{y^{2}}{16}\right)$$
or
$$- 4 p^{2} + \left(p y - y^{2}\right) = - \frac{15 y^{2}}{16} - \left(2 p - \frac{y}{4}\right)^{2}$$
General simplification
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$$- 4 p^{2} + p y - y^{2}$$
/ / ____\\ / / ____\\
| y*\1 - I*\/ 15 /| | y*\1 + I*\/ 15 /|
|p - ----------------|*|p - ----------------|
\ 8 / \ 8 /
$$\left(p - \frac{y \left(1 - \sqrt{15} i\right)}{8}\right) \left(p - \frac{y \left(1 + \sqrt{15} i\right)}{8}\right)$$
(p - y*(1 - i*sqrt(15))/8)*(p - y*(1 + i*sqrt(15))/8)
Assemble expression
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$$- 4 p^{2} + p y - y^{2}$$
$$- 4 p^{2} + p y - y^{2}$$
$$- 4 p^{2} + p y - y^{2}$$
Rational denominator
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$$- 4 p^{2} + p y - y^{2}$$
Combining rational expressions
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$$- 4 p^{2} + y \left(p - y\right)$$
$$- 4 p^{2} + p y - y^{2}$$
$$- 4 p^{2} + p y - y^{2}$$