Question

What is the answer? How do I create two equations?

Answer 1

The answer may be 36 but I am not very sure

Answer 2

let x = cost to rent a snorkel and y =  cost to rent a pair of fins

12x + 15y = 153
              x = 3y

substitute  x = 3y into 12x + 15y = 153

12x + 15y = 153
12(3y) + 15y = 153
36y + 15y = 153
51y = 153
y = 3

x = 3* 3 = 9

answer
It costs $9 to rent a snorkel