Final answer:
The GCF of 15 and 9 is 3. The original expression 15 - 9, when factored using the GCF, becomes 3(5 - 3).
Step-by-step explanation:
To find the greatest common factor (GCF) of two numbers, you need to list out all the factors of each number and find the largest number that is common to both. In the case of the numbers 15 and 9, the factors of 15 are 1, 3, 5, and 15, and the factors of 9 are 1, 3, and 9. The largest factor that these two numbers have in common is 3.
Therefore, the GCF of 15 and 9 is 3. When expressing the original expression using the GCF, you would divide each term by the GCF and use it to factorize the expression. Thus, the expression 15 - 9 can be written as 3(5) - 3(3), which factors out to 3(5 - 3).