Answer
$(3x+y)(9x^2+3xy+y^2)$
Work Step by Step
RECALL:
$A^3+B^3=(A+B)(A^2-AB+B^2)$
The given binomial can be written as:
$=3^3x^3+y^3
\\=(3x)^3+y^3$
The binomial above is a sum of two cubes.
Factor using the the formula above with $A=3x$ and $B=y$ to obtain:
$=(3x+y)[(3x)^2+(3x)(y) +y^2]
\\=(3x+y)(9x^2+3xy+y^2)$
