Sis a subset within a universal set, u. if s = {x, y, 4, 9, which could describe u? u = {keys on a keyboard}
u = {letters}
u = {numbers}
u = {punctuation marks}

Since U is a universal set where S is its subset, then set U should consist of all the letters, numbers and punctuation mark in a keyboard. In other words, universal set U consists of the following elements:
U = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
U = (x| x comprised of all the letters in the alphabet fro a to z)
U = (!,.,,,?,!:,;,",',-)