If Ann cleans in 12 hours then she cleans 

 of the apartment per hour. Similarly Liz cleans 

 of the appartment per hour. So together then can clean 

 of the apartment for hour. Therefor it will take them 

 hours to clean.
The general formula 

 can be used as a check if needed.
 
        
        
        
I hope this helps you 
7x-4x = -9-12
3x=-21
x= -7
        
             
        
        
        
If i am correct the answer should be 6.08