Mario's mistake is made when doing reciprocal. 
7 / (2/3) =
I cannot divide by 2/3, but if I do reciprocal, I can multiply by 3/2. The seven comes first, so it should not be flipped. 
Mario's mistake was when he flipped the 7 (or 7/1) to 1/7.
 
        
             
        
        
        
Answer:
5
Step-by-step explanation:
3² + 4² = 25
√25 = 5
use the formula 
 
        
             
        
        
        
Step-by-step explanation:
the explanation is pictured
 
        
             
        
        
        
Answer:
Choice D
Step-by-step explanation: