Please help with #13 and 14

Find ((S) for the set S = {a, 6} Which of the following candidates are binary or unary operations on the given sets? For those that are not, where do they fail? a) x compositefunction y = greatest common multiple of x and y; S = N b) x compositefunction y = x + y; S = the set of Fibonacci numbers. c) x# = the string that is the reverse of x; S = the set of all finite-length strings of symbols from the set {p, q, r} d) x compositefunction y = x + y; S = R