so one answer is 67, as you can transpose the numbers and add them, e.g. looking at the first row, you have 41 -transpose and it becomes 14, then add it to the other 12, but transpose it and you get 21, so 14 + 21 = 35, the number in the diamond. However
when I first looked at it, I thought of the numbers 67 or 76 could be the missing number. Why? Simple: take each of the numbers and reduce them to a single digit, i.e. taking the 3rd row down, we have 17 and 82 with 99 in the middle. By doing 1+7=8, and 8+2 = 10 = 1+0 =1,
we can then add the 2 single digits of 8 and 1 to get 9. Now 99 = 9+9 = 18 = 1+8 = 9.
Using that on the final row, we have 55=5+5=10=1+0=1, and 21 = 2+1=3, so adding the 2 single digits together, we get 1+3=4. So..... if we look at either 67 (6+7=13=1+3=4) or 76 (7+6=13=1+3=4) either could be the correct answer.
I wonder if there are more "answers" there as well!