\[\boxed{\text{685\ (685).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[x_{1}^{2} + x_{2}^{2} - 810 = 144\]
\[x_{1}^{2} + x_{2}^{2} = 954\]
\[x_{1}^{2} + x_{2}^{2} =\]
\[= x_{1}^{2} + 2x_{1}x_{2} + x_{2}^{2} - 2x_{1}x_{2} =\]
\[= \left( x_{1} + x_{2} \right)^{2} - 2x_{1}x_{2} =\]
\[= ( - p)^{2} - 2 \cdot 405 = 954\]
\[p^{2} - 810 = 954\]
\[p^{2} = 1764\]
\[p = \pm 42\]
\[Ответ:p = \pm 42\text{.\ \ }\]