Answer
$(3x+y)(x+y)$
Work Step by Step
$3x^{2}+4xy+y^{2}=$
two factors of $ac=+3$ whose sum is $b=4$ are $+3$ and $+1.$
Rewrite the middle term and factor in pairs.
$3x^{2}+4xy+y^{2}=3x^{2}+3xy+xy+y^{2}=$
$=3x(x+y)+y(x+y)$
$=(3x+y)(x+y)$