We start with the expression:
\[
9 \sqrt{56 x^{7} y^{12}}
\]
First, we factor \(56\) into its prime factors:
\[
56 = 4 \cdot 14 = 2^2 \cdot 14
\]
Thus, we can rewrite the expression as:
\[
9 \sqrt{4 \cdot 14 \cdot x^{7} \cdot y^{12}}
\]
Step 2: Separate Perfect Squares
Next, we separate the perfect squares from the non-perfect squares inside the square root:
\[
\sqrt{4} = 2, \quad \sqrt{x^{6}} = x^{3}, \quad \text{and} \quad \sqrt{y^{12}} = y^{6}
\]
This allows us to express the square root as:
\[
\sqrt{4 \cdot 14 \cdot x^{7} \cdot y^{12}} = \sqrt{4} \cdot \sqrt{14} \cdot \sqrt{x^{6} \cdot x} \cdot \sqrt{y^{12}} = 2 \cdot \sqrt{14} \cdot x^{3} \cdot y^{6} \cdot \sqrt{x}
\]
Step 3: Combine and Simplify
Now we can combine everything back together:
\[
9 \cdot 2 \cdot \sqrt{14} \cdot x^{3} \cdot y^{6} \cdot \sqrt{x} = 18 \sqrt{14} x^{3} y^{6} \sqrt{x}
\]
Final Answer
The simplified expression is:
\[
\boxed{18 x^{3} y^{6} \sqrt{14 x}}
\]