Variables that are the same are added so 4a and 2a are added making it 6a. Now you have 13+6a=3a+1. Now you have to move things. Move the 3a to the other side making it a negative. So now you have 13+6a-3a=1. You now have 13+3a=1. So move the 13 to the other side. 3a=13-1. Which makes 3a=12. All you have to do now is 12 divided by 3. So a=4
