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.
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.