Yes, you can add these square roots, and here's how to do it.
√6 + √24 = √6 + √4*6 = √6 + 2√6 = 3√6
You can add these squares because √24 can be written as square root of 4*6. As you already know, square root of 4 is 2, which means that you can add √6 and 2√6 and get 3√6.
