To simplify the expression $3\sqrt{15} + \sqrt{21} - \sqrt{35}$, let's look at each term:

1. None of the square roots can be simplified further because none of the numbers inside the square roots ($15$, $21$, or $35$) are perfect squares or have a common factor that is a perfect square.

2. Since all of the square roots are in their simplest form, the terms cannot be combined directly because they involve different radicals.

Thus, the expression remains:

$3\sqrt{15} + \sqrt{21} - \sqrt{35}$

This is the simplified form.

Would you like more details on why these roots can't be simplified?

Here are 5 questions related to this concept:

1. How do you simplify square roots involving perfect squares?
2. What does it mean for a square root to be in its simplest form?
3. How can you combine or add radical expressions?
4. What methods exist to estimate non-perfect square roots?
5. Can square roots with different radicands be factored in any way?

Tip: When simplifying square roots, always check if the number under the root has any perfect square factors.