\[x^{2} + 6xy + 8y^{2} =\]
\[= x^{2} + 6xy + 9y^{2} - y^{2} =\]
\[= (x + 3y)^{2} - y^{2} =\]
\[= (x + 3y - y)(x + 3y + y) =\]
\[= (x + 2y)(x + 4y)\]