It seems you've provided a polynomial expression, but there may be a misunderstanding or a miscommunication in the way it's structured. The expression you wrote is:

\[ x^3 - 6x^2y + 12xy^2 - 7y^3x^3 - 6x^2y + 12xy^2 - 7y^3 \]

Let's simplify the expression step-by-step. First, we will combine like terms:

1. Identify like terms:
- \( x^3 \)
- \( -6x^2y \) appears twice.
- \( 12xy^2 \) appears twice.
- \( -7y^3 \) appears once, but it seems to be \( -7y^3x^3 \), which doesn't include \( x \) raised to any power paired with \( y^3 \) under conventional polynomial structure.

2. Combine the like terms:

Starting with:
\[
x^3 - 6x^2y - 6x^2y + 12xy^2 + 12xy^2 - 7y^3x^3 - 7y^3
\]

Combining the coefficients for the like terms gives:
\[
x^3 - 12x^2y + 24xy^2 - 7y^3x^3 - 7y^3
\]

At this point, we can factor or reorganize further if needed:
- Notice \(7y^3\) and \(-7y^3x^3\) can be factored together. However, their combination hasn't a direct polynomial structure since they depend on \(x^3\) or it's additional \(y^3\).

Therefore, the simplified expression is:
\[
x^3 - 12x^2y + 24xy^2 - 7y^3x^3 - 7y^3
\]

If you have a specific goal in mind with this expression (like factoring, evaluating at certain values, or finding roots), please let me know!