You can use the midpoint formulae as I have in the picture below
Answer:
(-6,-3)
Step-by-step explanation:
go do to -6 then over to -3
Step-by-step explanation:

Customer #2 has the smallest total bill, including tax and tip.
Choice A, 1.06b + 0.16(1.06b), is correct.
Customer 3's bill is approximately 10% less than customer 1's bill.
Customer 2 leaves the bigger tip.
For customer #1:
(1.06(12.95))(1.15) = 15.79; the tip is 0.15(1.06(12.95)) = 2.06.
For customer #2:
1.06x = 11.65; x = 10.99; 20% tip = 0.2(10.99) = 2.20; total = 11.65+2.20 = 13.85.
For customer #3:
1.06(11.50) + 0.18(11.50) = 14.26; the tip is (0.18)(11.50) = 2.07.
Customer 3 spends 15.79-14.26 = 1.53 less than customer #1, which is 1.53/15.79 = 0.097 ≈ 10% less.