I am not sure if you can find an exact value since you are trying to compare 2 doubles and that's usually not a good idea, but you can get it using a tolerance.
it should be something like this:
% assuming you have 2 matrices x and y containing your data
z = x(y==6.585); % assuming that there is an exact value that equals it but most likely you don't so you need a tolerance
% this should work
tol = 10^-9;
z = x(abs(y-6.585) < tol); % you can also use find instead of x
