\[\boxed{\text{845.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (x + 3)^{3} - (x - 3)^{3} =\]
\[= x^{3} + 3x^{2} \cdot 3 + 3x \cdot 3^{2} + 3^{3} - \left( x^{3} - 3x^{2} \cdot 3 + 3x \cdot 3^{2} - 3^{3} \right) =\]
\[= x^{3} + 9x^{2} + 27x + 27 - x^{3} + 9x^{2} - 27x + 27 = 18x^{2} + 54\]
\[\textbf{б)}\ (a - 2b)^{3} + 6ab(a - 2b) =\]
\[= a^{3} - 3a^{2} \cdot 2b + 3a(2b)^{2} - (2b)^{3} + 6a^{2}b - 12ab^{2} =\]
\[= a³ - 6a^{2}b + 12ab^{2} - 8b^{3} + 6a^{2}b - 12ab^{2} = a^{3} - 8b³\]