f(2,3) = 10
f(7,2) = 63
f(6,5) = 66
f(8,4) = 96
The function f(x,y) defines a 3D surface. We are given four random points on that surface, and no other indication of what laws may govern this surface. It is visually obvious that you can fit any number of arbitrarily complex surfaces, described by all kinds of formulas, into those four points. That proves that there is no answer to this riddle.
That proves that there is no answer to anything. Cf. Wittgenstein ... the rascal.
"His friend Friedrich Waismann, who had spent much of the 1930s unsuccessfully attempting to co-author a book with Wittgenstein, eventually accused him of "complete obscurantism" ...
Use of the "+" isn't justified in the problem.
If you call a dog's tail a leg, how many legs does a dog have?
The answer is "four". A tail is not a leg, no matter what you call it.