Let us use prime factorization to find the prime numbers that make up 28:
28 = 2 x 14
2 is a prime number, 14 is not, so we have our first prime, 2.
14 = 2 x 7
We then further break the number down into two prime numbers, 2 and 7. Since 2 and 7 are prime and we can't break them down any further our answer is 2 x 2 x 7, or 2^2 x 7.
