[TEX]
\left\{\begin{matrix}
x = 1 & \rightarrow & a + b + c = 1 \\
x = -1 & \rightarrow & -a + b -c = 1 \\
x = -2 & \rightarrow & -8a + 4b - 2c = -2
\end{matrix}\right.
\rightarrow
\left\{\begin{matrix}
a = 1 \\
b = 1 \\
c = -1
\end{matrix}\right.
\rightarrow P = 1-3+5= 3
[/TEX]