### 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.