Factorize:

$a^{2}-b^{2}-4 a c+4 c^{2}=\left(a^{2}-4 a c+4 c^{2}\right)-b^{2}$

$=a^{2}-2 \times 2 a \times c+(2 c)^{2}-b^{2}$

$=(a-2 c)^{2}-b^{2}$

$=(a-2 c+b)(a-2 c-b)$