First we need to find a number that, when added to 7, will make 13 (3 plus 10, because since 3 is lesser than 7, we can't have a negative number in the empty space).
So, let's subtract 13 from 7:

So the third space is 6.
Now, in the middle column, we have 5 + 4 + 1 (the extra 1 comes from the 13 we found before), so we have 10, therefore the number in the middle empty space is 0.
Finally, we have 2 + missing number + 1 = 6, so the missing number is 6 - 2 - 1 = 3
For the explanation, we can write:
"7 plus a number is 13, so the number is 6. Then, 5 + 4 + 1 = 10, so the number is 0. Then, 2 plus a number plus 1 is 6, so the missing number is 6 - 2 - 1 = 3"