I'm assuming your teacher wants you to distribute. If so, then you multiply the outer term 8 by each term inside (3a and 5). After that you add.
8 times 3a = 24a
8 times 5 = 40
So 8(3a+5) becomes 24a+40
We do not combine 24a and 40 as they are not like terms. So we leave 24a+40 as it is.