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)

3(3a - 6b + 7c)

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