The difference between 15 and the product of 5 and y

In general, when you state "the difference between a point A and a point B" meaning that you have to take the final value at the point B subtract the initial value at the point A" 
For example: Find the difference in distance between a point A =2 m and a point B = 5 m? 
In this case: 
In general: The difference in distance = 5 m - 2 m = 3 m 

You can also write it as: 
The difference in distance = 2 m - 5 m = -3 m 
Since the distance between 2 points CAN'T be a negative value, you've to take the absolute value of -3 m to get the positive answer of 3 m. 
This based on a scalar quantity. 

Unless you're working on a Vector problem in Physics. In this case, you must take the final point subtract the initial point since the motion not only describes the magnitude but also describes the direction of the motion as well. 
Again, in this case, you can have the magnitude is always a POSITIVE number and the direction can be either POSITIVE or NEGATIVE depends on the direction that you chose as a positive. 

Back to your question. "the difference between 15 and the product of 5 and y" 

I would write this statement as 5y - 15