Example 7

$\;\;\;(4a + 7b)^2$

$=(A + B)^2 \text{, where } A = 4a \text{ and } B = 7b $

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

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

$=16a^{2} + 56ab +49b^{2} $