General simplification
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$$- 5 b^{2} - 7 b y - y^{2}$$
/ / ____\\ / / ____\\
| y*\-7 + \/ 29 /| | y*\7 + \/ 29 /|
|b - ---------------|*|b + --------------|
\ 10 / \ 10 /
$$\left(b - \frac{y \left(-7 + \sqrt{29}\right)}{10}\right) \left(b + \frac{y \left(\sqrt{29} + 7\right)}{10}\right)$$
(b - y*(-7 + sqrt(29))/10)*(b + y*(7 + sqrt(29))/10)
The perfect square
Let's highlight the perfect square of the square three-member
$$- 5 b^{2} + \left(- b 7 y - y^{2}\right)$$
Let us write down the identical expression
$$- 5 b^{2} + \left(- b 7 y - y^{2}\right) = \frac{29 y^{2}}{20} + \left(- 5 b^{2} - 7 b y - \frac{49 y^{2}}{20}\right)$$
or
$$- 5 b^{2} + \left(- b 7 y - y^{2}\right) = \frac{29 y^{2}}{20} - \left(\sqrt{5} b + \frac{7 \sqrt{5} y}{10}\right)^{2}$$
$$- 5 b^{2} - 7 b y - y^{2}$$
$$- 5 b^{2} - 7 b y - y^{2}$$
Rational denominator
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$$- 5 b^{2} - 7 b y - y^{2}$$
$$- 5 b^{2} - 7 b y - y^{2}$$
Combining rational expressions
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$$- 5 b^{2} + y \left(- 7 b - y\right)$$
$$- 5 b^{2} - 7 b y - y^{2}$$
Assemble expression
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$$- 5 b^{2} - 7 b y - y^{2}$$