Question

Factorise:
$q^2-10q+21$

Answer 1

$q^2-10q+21=q^2-7q-3q+21\left\{\begin{array}{cc}\because 21=(-7)\times (-3)& \\ -10=-7-3& \end{array}\right\}=q(q-7)-3(q-7)=(q-7)(q-3)$