For part a:
8 - (?) = 18
We need to find a number that subtracted from 8 gives us 18. That's number is -10, because:
8 - ( - 10) = 8 + 10 = 18
For part b:

We need to find a number such that -21 divided by that number is equal to -7, So, the missing number is 3, because:

For part c:
16 + ? = -36
We need to find a number that added to 16 give as -36. The missing number is -52, because:
16 + (- 52) = 16 - 52 = -36
For part d:
-6(?) = 66
We need to find a number that multiplies by -6 give us 66, that number is -11, because:
-6*(-11) = 66
For last part:
(?)^2=25
We need to find a number that to the power of 2 is equal to 25. The possible numbers are 5 and -5 because:

Answers: a. 18
b. 3
c. -52
d. -11
e. 5 and -5