Simplify $\left(3^{\left(2x-6\right)}\right)^{\left(9^{\left(x-3\right)}\right)}$ using the power of a power property: $\left(a^m\right)^n=a^{m\cdot n}$. In the expression, $m$ equals $2x-6$ and $n$ equals $9^{\left(x-3\right)}$