Answer:
2n - 9 = 17
Explanation:
We can allow n to represent the number. Twice the number can be represented by 2n.
Difference refers to subtraction and since we're told that it's the difference between twice the number and 9, we know that 9 is being subtracted form the number, which is how we get 2n - 9 = 17 and not 9 - 2n = 17.
You can see that the expression works by simply solving for n and looking back at the expression:
2n - 9 = 17
2n = 26
n = 13
The number is 13 and 13 twice is 26
The difference between 26 and 9 is 17