fx(3,2) = (f(3.5,2) - f(2.5,2))/(3.5 - 2.5) =
(22.4 - 10.2)/1.0 =
12.2/1.0 =
12.2
fx(3,2.2) = (f(3.5,2.2) - f(2.5,2.2))/(3.5 - 2.5) =
(26.1 - 9.3)/1 = 16.8

For fxy(3, 2), we have choices. If we don't care about centering the y, we can use the results already gathered here.
Then, fxy(3, 2) = (fx(3, 2.2)-fx(3,2))/(2.2-2) =
(16.8-12.2)/.2 = 23

If we had wanted to center it, we could have used
(f(3.5,2.2) - f(3.5,1.8)-f(2.5, 2.2) + f(2.5, 1.8))/((2.2-1.8)*(3.5-2.5)) =
(26.1 - 20 - 9.3 + 12.5)/(.4*1) = 23.25