IDNLearn.com is your go-to resource for finding answers to any question you have. Ask your questions and get detailed, reliable answers from our community of knowledgeable experts.
Answer:
Step-by-step explanation:
15 = 3 * 5
21 = 3 * 7
GCF = 3
15 + 21 = (3*5) + (3*7)
= 3*(5 + 7)