The expression provided is:

$-5 \sqrt{18}$

To simplify this expression, we start by simplifying the square root of 18:

$\sqrt{18} = \sqrt{9 \times 2} = \sqrt{9} \times \sqrt{2} = 3 \sqrt{2}$

Now substitute this back into the original expression:

$-5 \sqrt{18} = -5 \times 3 \sqrt{2} = -15 \sqrt{2}$

Thus, the simplified form of the expression is:

$-15 \sqrt{2}$

Would you like any further clarification or details?

Here are some related questions to expand on this topic:

1. How do you simplify square roots of larger numbers?
2. What are the general rules for simplifying radical expressions?
3. How can you add or subtract radical expressions?
4. What are the common mistakes when simplifying square roots?
5. How do you rationalize the denominator when dealing with radicals?

Tip: Always look for perfect squares when simplifying square roots to make the process easier.