In reality your common denominator should be the Least Common Multiple. The LCM is the product of highest occurring primes of the numbers prime factorizations...
4=2*2, and 12=2*2*3.  So the LCM is 2*2*3=12  Now that you know what the least common multiple is we can say that:
(3/4)(3/3)+5/12
9/12+5/12
(9+5)/12
14/12
7/6  which should be converted to a mixed number as this is an improper fraction...
(6+1)/6
1+1/6
1 1/6
        
                    
             
        
        
        
        
I would have to say that the answer is 3240 pounds of copper.
        
             
        
        
        
Step 1 because looking at PEMDAS he multiples the 4 and 2 before the 2 and the exponent.