Put this into slope intercept form. 
   y = 6 - x      becomes     y = -x + 6
"m" is the coefficient on x:   -1
"c" is the number without the x:   6
 
        
                    
             
        
        
        
a is given to us so just plug a into the first equation:
b-3 - 3b =9 
Add 3 to both sides:
b-3b=12
Combine like terms:
-2b=12
Divide by -2 to get b by itself:
b=-6
The only answer with b as -6 is the first one, (-9,-6)
 
        
                    
             
        
        
        
I think 90 im not sure tho i searched it up
        
             
        
        
        
Step-by-step explanation:
Amount of acid = 14.9% of 331 mL solution 
 = 0.149×(331 mL)
 = 49.3 mL acid
 
        
             
        
        
        
8n
Hope it helps, good luck