\[x^{2} - 8x - 3a = 0\]
\[D = 64 + 12a\]
\[64 + 12a < 0\]
\[12a < - 64\]
\[a < - 5\frac{1}{3}\]
\[Ответ:\left( - \infty;\ - 5\frac{1}{3} \right).\]