Factorise:

 By suitably arranging the terms:

$x^{2}-a x-b x+a b=x^{2}-b x-a x+a b$

$=\left(x^{2}-b x\right)-(a x-a b)$

$=x(x-b)-a(x-b)$

$=(x-b)(x-a)$

$\therefore x^{2}-a x-b x+a b=(x-b)(x-a)$