The perfect square
Let's highlight the perfect square of the square three-member
$$7 p^{2} + \left(- p 9 y - y^{2}\right)$$
Let us write down the identical expression
$$7 p^{2} + \left(- p 9 y - y^{2}\right) = - \frac{109 y^{2}}{28} + \left(7 p^{2} - 9 p y + \frac{81 y^{2}}{28}\right)$$
or
$$7 p^{2} + \left(- p 9 y - y^{2}\right) = - \frac{109 y^{2}}{28} + \left(\sqrt{7} p - \frac{9 \sqrt{7} y}{14}\right)^{2}$$
in the view of the product
$$\left(- \sqrt{\frac{109}{28}} y + \left(\sqrt{7} p + - \frac{9 \sqrt{7}}{14} y\right)\right) \left(\sqrt{\frac{109}{28}} y + \left(\sqrt{7} p + - \frac{9 \sqrt{7}}{14} y\right)\right)$$
$$\left(- \frac{\sqrt{763}}{14} y + \left(\sqrt{7} p + - \frac{9 \sqrt{7}}{14} y\right)\right) \left(\frac{\sqrt{763}}{14} y + \left(\sqrt{7} p + - \frac{9 \sqrt{7}}{14} y\right)\right)$$
$$\left(\sqrt{7} p + y \left(- \frac{9 \sqrt{7}}{14} + \frac{\sqrt{763}}{14}\right)\right) \left(\sqrt{7} p + y \left(- \frac{\sqrt{763}}{14} - \frac{9 \sqrt{7}}{14}\right)\right)$$
$$\left(\sqrt{7} p + y \left(- \frac{9 \sqrt{7}}{14} + \frac{\sqrt{763}}{14}\right)\right) \left(\sqrt{7} p + y \left(- \frac{\sqrt{763}}{14} - \frac{9 \sqrt{7}}{14}\right)\right)$$
General simplification
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$$7 p^{2} - 9 p y - y^{2}$$
/ / _____\\ / / _____\\
| y*\9 - \/ 109 /| | y*\9 + \/ 109 /|
|p - ---------------|*|p - ---------------|
\ 14 / \ 14 /
$$\left(p - \frac{y \left(9 - \sqrt{109}\right)}{14}\right) \left(p - \frac{y \left(9 + \sqrt{109}\right)}{14}\right)$$
(p - y*(9 - sqrt(109))/14)*(p - y*(9 + sqrt(109))/14)
$$7 p^{2} - 9 p y - y^{2}$$
$$7 p^{2} - 9 p y - y^{2}$$
Assemble expression
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$$7 p^{2} - 9 p y - y^{2}$$
$$7 p^{2} - 9 p y - y^{2}$$
Rational denominator
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$$7 p^{2} - 9 p y - y^{2}$$
Combining rational expressions
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$$7 p^{2} + y \left(- 9 p - y\right)$$
$$7 p^{2} - 9 p y - y^{2}$$