\[(a - 2b)(2a - 4b) =\]
\[= 2(a - 2b)(a - 2b) =\]
\[= 2(a - 2b)^{2} =\]
\[= 2\left( a^{2} - 4ab + 4b^{2} \right) =\]
\[= 2a^{2} - 8ab + 8b^{2}.\]