Firstly, it is important to understand what a prime number is. A prime number is one that is only divisible by 1 and itself. Thus, expressing 63 as a product of prime factors means putting it into a form whereby it is prime numbers multiplied together to give 63.To find the answer, you can use the complete factor tree method. In this method, you start with the original number 63 and express it as two products i.e. 7x9. Whenever you come across a prime number, you circle it so here 7 needs to be circled. Whenever you cannot circle a number, this means you can split it further into two products. Here 9 is not circled so you split it into 3x3. Now, since 3 is a prime number you need to circle both 3's. The final answer is all the circled numbers multiplied together. Thus, 63=3x3x7.