Missing number is 27.

How?
Number in middle column is sum of the respective numbers in first and third column. thus the missing number should be 56 – 29 = 27.

Answer is 36

Explanation:
1 + 2 => (1)^2 + (1*2) = 3
3 + 4 => (3)^2 + (3*4) = 21
5 + 6 => (5)^2 + (5*6) = 55
Thus 4 + 5 => (4)^2 + (4*5) = 36

Answer is 125

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