Compute the modulus and argument:
|-3 + √3 i| = √((-3)² + (√3)²) = √12 = 2√3
arg(-3 + √3 i) = π - arg(3 + √3 i)
… = π - arctan(√3/3)
… = π - arctan(1/√3)
… = π - π/6
… = 5π/6
So, the polar form is
-3 + √3 i = 2√3 (cos(5π/6) + i sin(5π/6))
