\[\boxed{\mathbf{312}.}\]
\[P(x) = M_{1}(x)(x + 3) + 10\]
\[P(x) = M_{2}(x)(x + 5) + 14\]
\[P(x) = M_{3}(x)\left( x^{2} + 8x + 15 \right) +\]
\[+ ax + b\]
\[x = - 3:\]
\[P( - 3) = 0 \cdot M_{1}(x) + 10 = 10;\]
\[P( - 3) = 0 \cdot M_{3}(x) - 3a + b =\]
\[= - 3a + b.\]
\[x = - 5:\]
\[P( - 5) = 0 \cdot M_{2}(x) + 14 = 14;\]
\[P( - 5) = 0 \cdot M_{3}(x) - 5a + b =\]
\[= - 5a + b.\]
\[\left\{ \begin{matrix} - 3a + b = 10 \\ - 5a + b = 14 \\ \end{matrix} \right.\ ( - )\]
\[2a = - 4\]
\[a = - 2.\]
\[b = 10 + 3a = 10 - 6 = 4.\]
\[Ответ:4 - 2x.\]