9. \( 5 \sqrt{98}-3 \sqrt{6} \cdot 5 \sqrt{3} \)

First, simplify \( 5 \sqrt{98} \). Since \( \sqrt{98} = \sqrt{49 \cdot 2} = 7\sqrt{2} \), we have: \[ 5 \sqrt{98} = 5 \cdot 7 \sqrt{2} = 35 \sqrt{2} \] Next, simplify \( -3 \sqrt{6} \cdot 5 \sqrt{3} \): \[ -3 \sqrt{6} \cdot 5 \sqrt{3} = -15 \sqrt{6 \cdot 3} = -15 \sqrt{18} \] Now simplify \( \sqrt{18} = \sqrt{9 \cdot 2} = 3 \sqrt{2} \), leading to: \[ -15 \sqrt{18} = -15 \cdot 3 \sqrt{2} = -45 \sqrt{2} \] Now combine the two terms: \[ 35 \sqrt{2} - 45 \sqrt{2} = (35 - 45) \sqrt{2} = -10 \sqrt{2} \] So the final answer is: \[ -10 \sqrt{2} \]