Simplify by combining like terms i need sadly
1) 5a-a
2) 5 divide 10s - 8s
3) 2a + 3b + 4a

Combining like terms is like looking at fruit.  You want to have all of the apples together and the oranges all together as well.  

1) 5a-a
5a is a simplified form of a+a+a+a+a  The coefficient "5" represents the number of "a" in the set.  If you were to take one "a" away from the group, you would decrease the coefficient by one.  5a-a=4a

2) 5 ÷ 10s-8s
Very similar to the problem above, only this time you are taking 8 away from the set of 10.  In doing so, you would have 2 "s" remaining.  These would be oranges compared to the 5 you have left over.  Since you can't combine the 5 with anything, the simplest form would be 5 ÷ 10s-8s= 5 ÷ 2s

3) 2a + 3b + 4a
This might take you down memory lane.  The Commutative Property can be applied to this in that you can change the order of the terms and the result will not be affected.  If you rewrite the equation to move like terms together, it goes from 2a + 3b + 4a to 2a + 4a + 3b.  Now, combine the variables that are common which would be 2a + 4a to create 6a.  Since you can't combine 3b with anything else, it will sit all by it's lonesome.  In the end, you have gone from  2a + 3b + 4a = 2a + 4a + 3b = 6a + 3b.

Hope this helps and it explains it well enough for you.  Good luck!