Seeking a maximum with respect to all the variables, you would find
In[11]:= FindMaximum[
x a - 2 x y b + 2 x c + y^2 d - 2 y e - f^2 + 2 r^2, {x, y, a, b, c,
d, e, f, r}]
During evaluation of In[11]:= FindMaximum::cvmit: Failed to converge to the requested accuracy or precision within 100 iterations. >>
Out[11]= {2.378459115707933*10^309, {x -> 1.23164*10^102,
y -> -1.79799*10^103, a -> 6.30468*10^102, b -> -1.4855*10^103,
c -> 1.26094*10^103, d -> 9.3925*10^102, e -> -1.39494*10^103,
f -> -1.39494*10^103, r -> 2.27695*10^103}}