5, 3, 7, 9 are all non-common factors
Practice, practice & practice.
I think the answer is 276
12 * 23 = 276
hope it helps
Steps:
18 x 2
What is that?
You can also think of it as 9 x 4 if that is easier
Then whatever you got for that, add 21 then subtract 12 and add 1
Answer:
D
Step-by-step explanation: