Use elimination. 
-x = -6 
X = 6 
-4 (6) + 4y = 12 
Y = 9 
(X,y) 
(6,9) is the solution
        
             
        
        
        
Answer:
-9x + 13
Step-by-step explanation:
9 -3x +(-6x) +4
you will add -3x+(-6x), then add 9+4
-3x+(-6x) = -9x   (a negative pus and negative = a negative
9 +4 = 13
-9x + 13
 
        
             
        
        
        
0.90 and I think .9 is another I'm not sure but I tried
 
        
                    
             
        
        
        
Yes you can and you do that by multiplying $42×1.30=54.60 and then $54.60×1.25=68.25