When you've got brackets, the first thing you have to do is to multiply everything outside of the bracket by everything inside of the bracket.
3(2a+5) becomes 3x2a=6a and 3x5=15. Therefore, this bracket becomes 6a+15.
5(a-2) becomes 5xa= 5a, and -2x5= -10.
You've now got to collect the terms that are the same.
5a+6a=11a
-10+15= 5.
therefore, expanded and simplified, this becomes 11a+5
Hope this is the explanation that you're looking for :)
