Lets assume it is the function in the form: f (x,y)= ax + by +c, a line in three dimensions.

Considering given information, we have:

f(3,4)=11 --> 3a +4b +c = 11

f(8,12)=63 --> 8a +12b +c = 63

f(9,6)=64 --> 9a+ 6b +c = 64

f(12,10)=?

Solving first 3 equations we find a= 18; b= - 55/2 ; c= 67 => f(x,y) = 18x - (55/2) y +67, then f(10, 12)= 180 - 330 +67 = **- 83**