Welll the y-intercept is negative 2 so on the y axis the first point is negative 2. 1/4 is the rise over run so you would go up from your first point one unit and over to the left four units or vise versa down one unit and over to the right four units, or the image
 
        
             
        
        
        
If the first roll is a three, then the only chance of the sum of the two numbers being less than six is if you roll a 1 or a 2. 
That is 2/6 chance, or simplified, a 1/3 chance. 
I hope this Helps!
        
                    
             
        
        
        
Mean number simply means average.