5+4=23 8+1=63 6+16=32 9+9=78 3+9=6 3+81=? - Vivendi Excel@ Mathematics Study Skills (71101) for PC, Mac

To find the answer, take the square of the first number and subtract from it the square root of the second number:
5 squared is 25
sgrt of 4 is 2; 25-2=23

9 squared is 81
sqrt of 9 is 3; 81-3=78

so:
3 squared is 9
sqrt of 81 is 9; 9-9=0

The answer is zero!