By DeMoivre's theorem,
(3 (cos(5π/6) + i sin(5π/6)))⁴ = 3⁴ (cos(4×5π/6) + i sin(4×5π/6))
… = 81 (cos(20π/6) + i sin(20π/6))
… = 81 (cos(10π/3) + i sin(10π/3))
… = 81 (cos(-2π/3) + i sin(-2π/3))
… = 81 (cos(2π/3) - i sin(2π/3))
… = 81 (-1/2 + √3/2 i )
… = -81/2 + 81√3/2 i
