Expanding the given expression, we get:

(y-3x)^2 = y^2 - 6xy + 9x^2
(x^2 + y)(3x - y) = 3x^3 - x^2y + 3xy - y^2

Adding both the expanded terms together, we get:

y^2 - 6xy + 9x^2 + 3x^3 - x^2y + 3xy - y^2

Rearranging the terms, we get:

3x^3 + 9x^2 - 6xy - x^2y + 3xy + y^2 - y^2

Combining like terms:

3x^3 + 9x^2 - 3xy

Therefore, the simplified expression is 3x^3 + 9x^2 - 3xy.