Which Variable Expression Corresponds To "6 Less Than A Number?

Say I have some number, say 42, and I want to talk about the number 6 less than that. I want to talk about 42 - 6 = 36.

Now suppose I have some different number, say 7, and I want to talk about the number 6 less than that. I want to talk about 7 - 6 = 1.

Now, put x where "some number" is. You see that I am always talking about x-6.