Example 90

$\;\;\;(7x + 10y)(7x - 10y)$

$=(A + B)( A - B) \text{ , where } A = 7x \text{ and } B = 10y $

$=A^2-B^2$

$=(7x)^2 - (10y)^2 $

$=49x^{2} - 100y^{2} $