Factorize each of the following expression:

$(3+2 a)^{2}-25 a^{2}$

$=(3+2 a)^{2}-(5 a)^{2}$

$=[(3+2 a)-5 a][(3+2 a)+5 a]$

$=(3+2 a-5 a)(3+2 a+5 a)$

$=(3-3 a)(3+7 a)$

$=3(1-a)(3+7 a)$