Find the GCF (greatest common factor) of the following terms.
\[
\{15,42\}
\]

Step 1 ：We are given two numbers, 15 and 42, and we are asked to find the greatest common factor (GCF) of these two numbers.

Step 2 ：The greatest common factor (GCF) of two numbers is the largest number that divides both of them without leaving a remainder.

Step 3 ：One way to find the GCF of two numbers is to factorize both numbers and multiply the common factors.

Step 4 ：Let's factorize the numbers. The factors of 15 are 1, 3, 5, and 15. The factors of 42 are 1, 2, 3, 6, 7, 14, 21, and 42.

Step 5 ：The common factors of 15 and 42 are 1 and 3.

Step 6 ：Among these common factors, the largest one is 3. Therefore, the GCF of 15 and 42 is 3.

Step 7 ：Final Answer: The greatest common factor (GCF) of 15 and 42 is \(\boxed{3}\).