Factorize the following:

The greatest common factor of the terms $9 x^{2} y$ and 3 axy of the expression $9 x^{2} y+3 a x y$ is $3 x y$.

Also, we can write $9 x^{2} y=3 x y \times 3 x$ and $3 a x y=3 x y \times a$.

$\therefore 9 x^{2} y+3 a x y=3 x y \times 3 x+3 x y \times a$

$=3 x y(3 x+a)$