Example 49

$\;\;\;(9x - 8y)^2$

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

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

$=(9x)^2 - 2(9x)(8y) + (8y)^2 $

$=81x^{2} - 144xy +64y^{2} $