Factorise:

We have:

$(3 x-4 y)^{2}-25 z^{2}=(3 x-4 y)^{2}-(5 z)^{2}$

$=\{(3 x-4 y)+5 z\}\{(3 x-4 y)-5 z\}$

$=(3 x-4 y+5 z)(3 x-4 y-5 z)$

$\therefore(3 x-4 y)^{2}-25 z^{2}=(3 x-4 y+5 z)(3 x-4 y-5 z)$