Example 45

$\;\;\;(9a - 8b)^2$

$=(A - B)^2 \text{, where } A = 9a \text{ and } B = 8b $

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

$=(9a)^2 - 2(9a)(8b) + (8b)^2 $

$=81a^{2} - 144ab +64b^{2} $