28: 1, 2, 4, 7, 14, 28
40: 1, 2, 4, 5, 8, 10, 20, 40
The common factors are 1, 2, and 4.
You can use a factor rainbow to help you find the factors (:
C = 2pi r
C/2 Pi = r.
I believe this would be the solution.
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.
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