CB = (-5-4, 2-(-4), 6-(-3)) = (-9, 6, 9)
AC = (4-4, -4-6, -3-3) = (0, -10, -6)
BA = (4-(-5), 6-2, 3-6) = (9, 4, -3)
a = 2*(-9,6,9) + (0,-10,-6) -3*(9,4,-3) = (-18,12,18) + (0,-10,-6) - (27,12,-9) = (-45, -10, 21)
угол С = (AC*CB)/( |AC|*|CB|)
|AC| = sqrt(0+(-10)^2+(-6)^2) = sqrt(136) =4sqrt(34)
|CB| = sqrt((-9)^2+6^2+9^2) = sqrt(198) = 3 sqrt (22)
AC*CB = 0-60-54 = -104 <0 => угол тупой.
угол C= -104/(4sqrt(34)*3sqrt(22))