If the standard form is a + bi then the trigonometric form is r*(cos theta + i*sin theta) where
r = sqrt(a^2 + b^2) and theta = arctan(b/a)
So, for 4 (squareroot) 3 - 4i we have r = sqrt(16*3 + 16) or r = 8
theta = atan(-4/4*sqrt(3)) = -30°
4 (squareroot) 3 - 4i → 8(cos(30°) - i*sin(30°))