Explain why 5 is a composite number

Well for starters 5 isn't a composite number, it's a prime number which I assume you are talking about and 5 is a prime number because it cannot be divided by anything but 1.

A prime number has only two factors: 1 and itself. A composite number has more than two factors. The number 1 is neither prime nor composite. The prime numbers between 2 and 31 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 and 31 since each of these numbers has only two factors, itself and 1.