Okay so you want to make all of the fractions have a common denominator. Think of the lowest number that 3 and 5 go into, the answer you should get is 15. So 15 is the common denominator.
Now we're going to make 1/3 have a denominator of 15. 1/3 of 15 would be 5. You get that by figuring out what you have to multiply 3 by to get 15 which is 5. Then you multiply 5 and 1 and you get 5. The answer you will get will be 5/15 which is equal to 1/3.
Next we're going to make 4/5 have a denominator of 15. 4/5 of 15 would be 12/15. You can get this by figuring out what you have to multiply 5 by to get 15 which is 3. Then you take 3 and multiply it by 4 to get that 12 in 12/15. The answer 12/15 is equal to 4/5 of 15.
So now you have 5/15 + a = 12/15.
The denominator of the fraction that "a" will be is 15 because that is what we made the common denominator.
So what we have is 5/15 + x/15 = 12/15.
Think of what you have to add to 5 to get 12. (The answer is 7)
So a is equal to 7/15
Your final answer should be 5/15 + 7/15 = 12/15