+128408 Not the first one
(3x^2 - y)^2 = (3x^2 - y) (3x^2 - y) = 9x^4 - 3x^2y - 3x^2 + y^4 =
9x^4 - 6x^2 + y^4
Second one (3x^2)^2 - y^2 = 9x^4 - y^2
Third one 9(x^2)^2 - y^2 = 9x^4 - y^2
Not the fourth one (9x^2)^2 - y^2 = 81x^4 - y^2
Fifth one (3x^2 + y) (3x^2 - y) = 9x^4 +3x^2y - 3x^2y - y^2 = 9x^4 - y^2
So......First, Third, Fifth
