\[y = x^{3} - x^{2} - 7x + 6;\ M(2;\ - 4):\]
\[k = y^{'}(x) = 3x^{2} - 2x - 7 =\]
\[= 3 \bullet 4 - 4 - 7 = 12 - 11 = 1.\]
\[a = arctg\ k = arctg\ 1 = \frac{\pi}{4}.\]
\[Ответ:\ \ a = \frac{\pi}{4}\text{.\ }\]