Example 52

$\;\;\;(7a - 2b)^2$

$=(A - B)^2 \text{, where } A = 7a \text{ and } B = 2b $

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

$=(7a)^2 - 2(7a)(2b) + (2b)^2 $

$=49a^{2} - 28ab +4b^{2} $