The answer to this question is:
y = -2x - 1
 
        
             
        
        
        
Where is the picture of the triangle?
        
                    
             
        
        
        
In such type of questions, all that you are supposed to do is, use basic mathematics to eliminate any one of the given two variables.
Check out the equations if in case you can multiply any of the given ones with a -1 and add the two equations,eliminating one of the variables.
        
                    
             
        
        
        
-2 1/2 / 6 would be -5/12, just turn - 2 1/2 into a decimal and divide by 6 :)
        
             
        
        
        
2 pieces of squid = $3.50 
Therefore 1 piece of squid = $1.75 (3.50÷2)
1 piece of eel = $3.25
3 pieces of tuna = $6.75
Therefore 1 piece of tuna = $2.25 (6.75÷3)
4 pieces of crab = $8.00 
Therefore 1 piece of crab = $2 (8÷4)
Put them in order of price and you get 
$1.75 , $2 , $2.25 , $3.25
squid,crab,tuna,eel