So, we could say something like...
2 x ____ = 16 and ask you to fill in the blank. The answer would be 8, right?
Instead of a blank, algebra uses letters.
2 x n = 16. What is n? Still 8, right?
Now, instead of 2 x n, we just say 2n. It's like saying 2 Sams. Sam plus Sam = 2 Sams, but note that we don't say 1 Sam plus 1 Sam, because that would be silly.
Twice a number means two times a number. Pick a letter. Usually we use x, but we can use n for "number."
Twice a number = 2x or 2n or whatever.
The total of 15 and twice a number. 15 + 2x.
This is not 17 or 17x because the 15 has no variable. The variable, x in this case is an unknown number which we can assign a value, though that's for a later time.
x = 5.
15 + 2 x 5 = 15 + 10 = 25, which is not 17 or 17(5), right?
x = 7
15 + 2 x 7, etc.
Hope this helps!