If a and b have a common non-square-number factor**, and the square root of the 2 cannot be simplified into a whole number, then √a and √b can be added together!!
Your question is √5+√8, because 5 and 8 does not have a common non-square-number factor**, they cannot be added together. The answer is:
√5+√8=√5+√22⋅2=√5+√22⋅√2=√5+2√2
Let's say √8+√32, 8 and 32 have a common non-square-number factor** 2, so they can be added together. The answer is:
√8+√32=√22⋅2+√24⋅2=√22⋅√2+√24⋅√2=2⋅√2+22⋅√2=2√2+4√2=6√2
**: common non-square-number factor here means common factor that are not square numbers(i.e. 1,4,9,16,25,36,49,.....)