[tex][6(\cos 70^{\circ} + i \sin 70^{\circ})]^3\\\\=6^3[ \cos ( 3 \times 70)^{\circ} + i \sin ( 3 \times 70)^{\circ}]\\\\=216 [\cos(210^{\circ}) + i \sin( 210^{\circ})]\\\\=216[\cos(2\times 90^{\circ} + 30^{\circ}) + i \sin( 2 \times 90^{\circ} + 30^{\circ})]\\\\=216[-\cos 30^{\circ}-i\sin 30^{\circ}]\\\\=216\left[-\dfrac{ \sqrt 3} 2 - i \cdot \dfrac 12 \right]\\\\=-108\left(\sqrt 3 +i \right)[/tex]
