You're almost there. When you call string on a non-scalar double array, each element of the double array becomes an element of the string array.
n = [3.2,4.5,2.5,1.2,7.2];
s = string(n)
The + operator when given two compatible string arrays concatenates them element-wise.
part1 = string(1:numel(n)) + ". "
If one of the inputs to + is a string array and the other a compatible double array, the double is converted to a string to reduce this to a previously solved problem (two compatible string arrays.)
part2 = part1 + n + " Hz"
No fprintf needed, though if you're doing this as part of an assignment that requires it:
fprintf('%s\n', string(1:numel(n)) + ". " + n + " Hz")
