Cost of chicken wings at Buffalo Bills = 8 wings for $7
Cost of 1 wing at Buffalo Bills = 
Cost of chicken wings at Buffalo Mild Wings = 12 wings for $10
Cost of 1 wing at Buffalo Mild Wings = 
Cost of chicken wings at Wingers = 20 wings at $17
Cost of 1 wing at Wingers = 
Hence, comparing all the three costs per wing, we can see that Buffalo Mild Wings is serving chicken wings at lowest price of $0.833 per wing.
The double implication "if and only if" is graphically represented by an arrow pointing in both direction, so the correct writing is

Since you are subtracting 11 on the side with the variable, you have to add it on the other side. 17+11=28. Then, to solve the problem you are left with, 7c=28. You are multiplying on the side of the variable, so you divide 28 by 7. Your answer to the problem is C=4 i hope this helps