Combine any like terms in the expression. If there are no like terms, rewrite the expression.

To combine like terms in the expression \(3g^2h^2 - 3g^2h^2\), we can see that the two terms are identical but with opposite signs.

When we combine them:

\[ 3g^2h^2 - 3g^2h^2 = 0 \]

So the simplified expression is:

\[ 0 \]