\[\boxed{\mathbf{304}.}\]
\[P(x) = x^{5} - x^{4} + cx^{3} =\]
\[= x³(x^{2} - x + c)\]
\[1)\ Q(x) = \ x + 4 \rightarrow x = - 4:\]
\[( - 4)^{2} - ( - 4) + c = 0\]
\[c = - 20.\]
\[2)\ Q(x) = x - 5 \rightarrow x = 5:\]
\[5^{2} - 5 + c = 0\]
\[c = - 20.\]