the elements of 'b' given in the question are not actually present in the matrix 'a', so I used values in b from 'a'
a = [
0.855750000000000 0.858420000000000 0.858110000000000 0.858210000000000 0.858880000000000
0.857680000000000 0.858700000000000 0.858920000000000 0.858480000000000 0.858630000000000];
b = [0.85811
[~,c] = find(a==b);
Note that, if the values are not exactly the same, then you will need to do the comparison with a tolerance value like this
[~,c] = find(abs(a-b)<1e-6);