? = 6
If you add up the 3 digits in each row and column, the sum of each adds up to 15! Thinking about the missing number, the row it's in so far adds up to 9 (7+2=9). And the column it's in adds up to 9 (8+1=9). So what's the missing number? Well, what number added to 9 equals 15? 6!!