Answer is 10
How?
There is gap of 4 between numbers
8+4 = 12
3+4 = 7
9-5 = 4
simillarly
?-4 = 6=> ?=10
NOTE: there is a better and shorter explanation(suggested by Guestlogic in comment), opposite numbers sum is 15, that’s why answer should be 15 – 5 = 10
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Guestlogic says
The answer is 10 but it is because oppostte numbers add up to 15.