To simplify the expression $2 \sqrt{20} + 3 \sqrt{18}$, we will first break down the square roots into simpler terms.

1. Simplify $\sqrt{20}$: $\sqrt{20} = \sqrt{4 \times 5} = \sqrt{4} \times \sqrt{5} = 2\sqrt{5}$ So, $2 \sqrt{20} = 2 \times 2 \sqrt{5} = 4 \sqrt{5}$.

2. Simplify $\sqrt{18}$: $\sqrt{18} = \sqrt{9 \times 2} = \sqrt{9} \times \sqrt{2} = 3 \sqrt{2}$ So, $3 \sqrt{18} = 3 \times 3 \sqrt{2} = 9 \sqrt{2}$.

Now, the expression becomes: $4 \sqrt{5} + 9 \sqrt{2}$

This is the simplified form of the given expression. Since the square roots are of different numbers, they cannot be combined further.

Would you like any further explanation on this or have any questions about simplifying square roots?

Here are some related questions you might find useful:

1. How do you simplify square roots of larger numbers, like $\sqrt{72}$?
2. How can you add or subtract square roots that involve the same number inside the radical?
3. What are some methods for estimating square roots without a calculator?
4. How do you solve expressions involving both square roots and cube roots?
5. How do you work with radicals in algebraic equations?

Tip: When simplifying square roots, always look for perfect squares that can be factored out of the radical, as this helps simplify the expression faster.