√8 + √8 = 2√8
Add all the radicals with similar numbers inside together. Start by first simplyfying each to the most simplest form. From there, you can add each "same-radical-numbers" together.
For example
10√3 + 5√3 + 9√2
10 + 5 = 15
15√3 + 9√2 would be final answer (since √3 & √2 are two different radicals, do not add them together)
hope this helps