Verified answer

Step-by-step explanation:

[tex] \\ \rm3xy + 15 {xy}^{2} + {9xy}^{2} + {21y}^{3} [/tex]

[tex] \\ \rm = \red{ \boxed{ \rm3y}}(x + 5xy + 3xy + 7 {y}^{2} )[/tex]

The common factor in 3xy + 15xy² + 9xy² + 21y³ is 3y.