First, replace all variables with the given values. 
12(9) + 9(2) + 2(12) 
This should then equal 150, then the formula says to multiply this answer by 2. Doing so, should give a final answer of 300.
        
             
        
        
        
Answer:
The last one.
Step-by-step explanation:
 
        
             
        
        
        
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➷ The nth term would be x3
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For the first one it is the fourth one 15/x