Let's review the terminology. A product is what you get when you multiply two numbers. So, the product of 5 and 2 is basically "5*2," and the product of 5 and 1 is "5*1."
The difference means when you subtract one from the other. So, the difference of 5*2 and 5*1 means "(5*2)-(5*1)". (Note: I put in parentheses to make sure the reader understands that 5*2 is separate from 5*1. 5*2-5*1 is confusing: you might not know what to do first!)
So, "(5*2)-(5*1)" can be your answer, OR you can simplify it further by actually doing the multiplication: "10-5". If you want the answer, just subtract--the expression is equal to 5.
Answer: "(5*2)-(5*1)" OR "10-5" OR "5"