There are many ways to express NUMBERS.
10 · ( 3 – 2 ) = 10 ------
15 ÷ 5 = 3 ------ Single Other Number
4 + 13 = 17 --------
Above are numerical expressions. What does that mean? You have two or more numbers with certain operations (like +, -, multiplication... and more) that combine these numbers so that they can convert into a single other number.
9 – n
The above is an Algebraic expression. An algebraic expression has a variable (like "n") and it does not have a single other number (unless stated or can be solved...you will learn this later in the course). But what you need to know here is that if I were to make n = 1, then the algebraic expression would convert into a numerical expression and the single other number would be 8. But I can also change my assumption and say that n=2 and then my single other number would be 7. This answer is different from 8. So an algebraic expression can change unlike the numerical expressions that surround it in your original question.