Answer
$(y-1+3x)(y^2-2y+1-3xy+3x+9x^2)$
Work Step by Step
Using $a^3+b^3=(a+b)(a^2-ab+b^2)$ or the factoring of two cubes, then,
\begin{array}{l}
(y-1)^3+27x^3
\\=
[(y-1)+(3x)][(y-1)^2-(y-1)(3x)+(3x)^2]
\\=
[y-1+3x][(y^2-2y+1)-3xy+3x+9x^2]
\\=
(y-1+3x)(y^2-2y+1-3xy+3x+9x^2)
.\end{array}