OK first off so difference is the result of subtracting to numbers or values from each other.
So in this case to set up we would do the two things the problem is asking and then subtract them from each other.
The product of 17 and a number (x) would be written as 17x since a product is the result of multiplying two values. Twice the number would be written as 2x because the number (x) twice is just 2 multiplied by the number.
So, if you want to find the difference between the two, just subtract 2x from 17x. This will leave you with 15x. Good luck hope this helped!