Factorise:

We have:

$63 a^{2}-112 b^{2}=7\left(9 a^{2}-16 b^{2}\right)$

$=7\left\{(3 a)^{2}-(4 b)^{2}\right\}$

$=7(3 a+4 b)(3 a-4 b)$

$\therefore 63 a^{2}-112 b^{2}=7(3 a+4 b)(3 a-4 b)$