Example 2

$\;\;\;(3x + y)^2$

$=(A + B)^2 \text{, where } A = 3x \text{ and } B = y $

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

$=(3x)^2 + 2(3x)(y) + (y)^2 $

$=9x^{2} + 6xy +y^{2} $