Ответ:

Х=3

216-8х куб=72х-8х куб

72х=216

х=3

Verified answer

[tex]\displaystyle\bf\\\underbrace{(6-2x)(36+12x+4x^{2}}_{6^{3} -(2x)^{3}} =72x-8x^{3} \\\\\\216-8x^{3}-72x+8x^{3}=0\\\\\\72x=216\\\\\\x=3[/tex]