I'm sending you two screenshots of the explanation I hope this helps.
Answer:
the answer to your question is A
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.
The correct answer among the choices listed is option C. The statement that is not true about corresponding sides is that they are connected by a vertex. Corresponding sides are not connected, they are separate parts in similar polygons.
