a = -4/(1 - i * sqrt 3) = -4 * (1 + i * sqrt 3)/((1 - i * sqrt 3) * (1 + i * sqrt 3))
(x - iy) * (x + iy) = x^2 + y^2
Тогда
a = -4 * (1 + i * sqrt 3)/(1 + 3) = -1 - i * sqrt 3