Let's think about like Mark starts off with $36. Take away $22 for that t-shirt he bought. Now he has $14. He uses $4 for water, now he has $10. The rest of his money is spent on tacos. $10/2 tacos is $5 for each taco. :)
The equation is simply 8s where s is the length of a side. There are 8 sides. You are just adding the length of all 8 sides together to get the perimeter.
