\[\boxed{\text{844\ (844).\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[если\ x = 2:\ \ \ \ \ \ \]
\[{\ a}^{2} - 2 \cdot 2 = a^{2} - 4 =\]
\[= (a - 2)(a + 2);\]
\[если\ x = 8:\ \]
\[\ a^{2} - 2 \cdot 8 = a^{2} - 16 =\]
\[= (a - 4)(a + 4);\]
\[если\ x = 18:\ \]
\[a^{2} - 2 \cdot 18 = a^{2} - 36 =\]
\[= (a - 6)(a + 6).\]