Based on the given illustrative examples, how do we add radicals? How do we subtract radicals?

"There are two keys to combining radicals by addition or subtraction: look at the index, and look at the radicand. If these are the same, then addition and subtraction are possible. If not, then you cannot combine the two radicals.

Making sense of a string of radicals may be difficult. One helpful tip is to think of radicals as variables, and treat them the same way. Let’s start there."