To solve the given problem, we need to factorize the expression $7 + 21$ as a product of two factors.

First, we can rewrite the expression: $7 + 21$

We can factor out the common factor, which is 7: $7 \times 1 + 7 \times 3$

Next, we use the distributive property to factor out the 7: $7(1 + 3)$

Therefore, the correct expression that shows $7 + 21$ written as a product of two factors is: $7(1 + 3)$

Among the given choices:

• $7(1 + 3)$
• $3(1 + 7)$
• $3(3 + 7)$
• $7(3 + 3)$

The correct answer is: $7(1 + 3)$

Would you like further details or have any questions?

Here are 5 related questions to further your understanding:

1. How do you identify the greatest common factor (GCF) of two numbers?
2. Can you factorize the expression $14 + 35$ in a similar way?
3. What is the distributive property and how is it used in factoring?
4. How can you factorize the expression $21 + 28$ using the GCF?
5. Why is it important to recognize common factors in algebraic expressions?

Tip: Always look for the greatest common factor when trying to factorize expressions. This can simplify the process and help you identify the factors more easily.