The expression 3x^3 - 6x + 9x^2 can be simplified by combining like terms.

The like terms in this expression are the terms with the same variable and exponent.

The terms 3x^3 and 9x^2 have the same variable (x) and exponent (2 and 3, respectively), so they can be combined.

3x^3 + 9x^2 = 12x^2

The simplified expression is 12x^2 - 6x.