Just list the factors of 24. Those are the possible numbers of students that can be in each row. 
1,2,3,4,6,8,12,24 are the possible row sizes.
        
             
        
        
        
Add the 3 item totals, then divide that number by 100, then, take that number and multiply by 9 (this gets the tax total). then you are going to want to add the total of the 3 items, and the tax amount, and you have your answer. do this with all the peoples items :)
        
                    
             
        
        
        
The slope is 0 and the equation is y = -2
        
                    
             
        
        
        
The answer is all real numbers because -2x-6=-2x-6