Example 86

$\;\;\;(7a + 4b)(7a - 4b)$

$=(A + B)( A - B) \text{ , where } A = 7a \text{ and } B = 4b $

$=A^2-B^2$

$=(7a)^2 - (4b)^2 $

$=49a^{2} - 16b^{2} $