Question 1081960
This looks as though its from an Algebra I class (maybe pre-Algebra?).  As such I will explain it in more detail than I would otherwise:

For  a+a = ?     how many a's are there?    There are two, right?   You can write a+a  as  '2*a'  (two times a) which is shortened to just {{{ highlight(2a)}}}.   So all you need to do is count how many a's there are and write the number in front of a single a.   

a+2a = ?     This is just one a plus two a's which gives you three a's, so {{{highlight( 3a)}}}.
   
What do you think happens when you have a's and b's ?    Just do the above process on the a's and the b's separately and put the + sign between the two results.    This process is calls "combining like terms" extends to other expressions as well.

I will do one more and leave the last one for you:
a + 3a + b + 4a + b = 
(a + 3a + 4a) + (b + b) =     (this step can be done in your head as you do a few of these)
{{{ highlight(8a + 2b)}}}

Send a message if you need help.