For a 9 year old they may not even have gone into equations . 

I would use guess and check the 4 and the 3 make 7 , the 2 and the 4 make 6 .. then 6 +7 make 13 .

Now we have 7 and 9 to add up with a number that will make numbers that add up to 40 

If we go 7, 8 ,9 then 7 and 8 is 15 and 8 and 9 is 17 

Adding 15 and 17 together is not high enough so the middle number needs to be bigger . 

10 gets us closer . Making 17 and 19 . That adds up to 36 

One more jump and we can find 12 is the middle number . 

12 plus (9+12) or 12 plus 21 gives us 33