What is the difference between 2x and 2n?

x denotes an unknown amount in an expression while n denotes where a term is in a sequence.

2x: a sequence goes 1,2,3 and 4 the term "n" is acertain amount.

2n: This is how they are usually used. but they can also be interchangeable if we want to use them interchangeably. x and n both means an "unknown value".