AUTO ANSWER (AFTER SEVERAL DAYS): 

This is not a hard question, but many can't solve it. Because they thought: "The question is wrong!". Exactly, 99 = 72 + 27; 45 = 27 + 18; 39 = 18 + 21; So the ?? must be 15. But why does 21 = 13 + 7? We have gone into the wrong MINDWAY.  Let's think of another way. We have 9 + 9 + 7 + 2 = 27 (Sum of the digits in line 1, row 1 + sum of the digits in line 2, row 1 = line 2, row 2). Because 2 + 8 + 1 + 2 = 13 and 3 + 6 + 2 + 1 = 12 so the answer correct is 12. 

(#560 - 1000 best IQ questions in history)

AUTO ANSWER (AFTER SEVERAL DAYS): 

This is not a hard question, but many can't solve it. Because they thought: "The question is wrong!". Exactly, 99 = 72 + 27; 45 = 27 + 18; 39 = 18 + 21; So the ?? must be 15. But why does 21 = 13 + 7? We have gone into the wrong MINDWAY.  Let's think of another way. We have 9 + 9 + 7 + 2 = 27 (Sum of the digits in line 1, row 1 + sum of the digits in line 2, row 1 = line 2, row 2). Because 2 + 8 + 1 + 2 = 13 and 3 + 6 + 2 + 1 = 12 so the answer correct is 12. 

(#560 - 1000 best IQ questions in history)

Read the P/s: It's 7, not 8

Because you type incorrectly!

Sorry, wrong answer. Why does 21 = 13 + 7?

We have:

99 = 72 + 27

45 = 27 + 18

39 = 18 + 21

36 = 21 + ??

28 = ?? + 13

21 = 13 + 7

(The number on the first row = The number below it + The number right next to the number below it)

So ?? is 15