Explain why 3 × 5 × 7 + 7 is a composite number.

Composite numbers are the numbers which have more than two factors. 

The given numbers is 3 x 5 x 7 + 7.

The given number is composite number that needs to be explained.

The given number can be simplified as

3 x 5 x 7 + 7 = 15 x 7 + 7

= 105 + 7 = 112

Breaking the number obtained as the product of its factors,

112 = 2 x 2 x 3 x 11

The number has more than two factors, so it is a composite number.

The given number is a composite number as it has more than two factors.