Step-by-step explanation: The chart I attached shows points that are plotted on the proportional line. In order to find the constant of proportionality from these points, you have to divide the amount of change by the number of years. For example, 3 divided by 10 or 6 divided by 20. This results in 0.3, which means that every year, there is 0.3 centimeters of change in the sea level.
The student should have phrased it to be "the difference between the product of 5 and y and 15", as the way the student writes it implies that 5y is being subtracted from 15 instead of the other way around.