Answer:
3(3a - 6b + 7c)
Step-by-step explanation:
Write each term as a multiplication of its factors. Then see which factors are in common to all terms.
9a - 18b + 21c =
= 3 * 3 * a - 2 * 3 * 3 * b + 3 * 7 * c
The common factors are shown below in bold:
= 3 * 3 * a - 2 * 3 * 3 * b + 3 * 7 * c
The only common factor is 3.
Now use the distributive property to factor out a 3.
= 3(3 * a - 2 * 3 * b + 7 * c)
Now multiply each term in parentheses again.
= 3(3a - 6b + 7c)
