You want
position = find(a==5);
Your example is maybe not the best, since the position and the value are the same in every case, but just to be clear on what you mean:
x = [2 5 7 4 3];
position = find(x==5)
will return position = 2, because 5 is in the 2nd position in x.
FYI, in the syntax you used in your example, MATLAB was returning the positions of the first 5 non-zero entries of a.
