There are several choices to find minimum/maximum values. You can take a look at the built-in function fmincon for example. In your case, if the data you present is the data, you can do something like this:
% find the maximum value in the output array:
[y_max max_indes] = max(y);
a_max = a(max_index); % get the corresponding value from the _a_ array
b_max = b(max_index); % get the corresponding value from the _b_ array
