Example 51

$\;\;\;(5p - 7q)^2$

$=(A - B)^2 \text{, where } A = 5p \text{ and } B = 7q $

$=A^2 - 2AB + B^2$

$=(5p)^2 - 2(5p)(7q) + (7q)^2 $

$=25p^{2} - 70pq +49q^{2} $