9a-18b+21c apply distributive property to factor out gcf of all three terms

The GCF of the three terms (9a, - 18b and 21c) is 3

Rewrite each of the terms so 3 is a factor

9a = 3*3a

-18b = - 3*6b

21c = 3*7c

So we can say ...

9a - 18b + 21c = 3*3a - 3*6b + 3*7c

9a - 18b + 21c = 3 (3a - 6b + 7c)

Answer: 3 (3a - 6b + 7c)

If you distribute outer 3 to each of the inner terms and multiply, you'll get the original expression again.