First, we need to find what 12*2 is.
12*2=24
Next,we need to find out what 9*2 is.
9*2=18
24=18=42.
Finally, we need to find out which of the following equals 42.
21*2=42, 21*4=84, 22*2=44, and 22*4=88
21*2=42,so that is your answer.
Hope this helped