Final answer:
The equation that relates y and x when they are directly proportional is y = 9x.
Step-by-step explanation:
The relationship between x and y is described as directly proportional. When x is 15, y is 135. To find the equation that relates y and x, we can use the formula y = kx, where k is the proportionality constant. Substituting the given values, we have 135 = k(15). Solving for k, we get k = 9. Hence, the equation that relates y and x is y = 9x.