Factorise:

We have:

$63 a^{2} b^{2}-7=7\left(9 a^{2} b^{2}-1\right)$

$=7\left\{(3 a b)^{2}-(1)^{2}\right\}$

$=7(3 a b+1)(3 a b-1)$

$\therefore 63 a^{2} b^{2}-7=7(3 a b+1)(3 a b-1)$