For this question we can think of a and b as different numbers just for the sake of making the question easier. For example "a" could be 2 and "b" could be 3 (a=2 and b=3).
(However, keep in mind that "a" and "b" can be any number and that we don't know their values. But we can assign some values to them in order to solve the question more easily)
So if a=2 and b=3 in our example the notation for their sum would be 2+3. If we just put a for 2 and b for 3 we get a+b, which is the answer
