\[5b^{2} - (a - 2b)^{2} =\]
\[= 5b^{2} - \left( a^{2} - 4ab + 4b^{2} \right) =\]
\[= 5b^{2} - a^{2} + 4ab - 4b^{2} =\]
\[= b^{2} - a^{2} + 4ab\text{.\ }\]