How is subtracting - 4x from 9x similar to subtracting - 4 from 9?

When you have two variables which in this case is X, they are the same and you would treat it the same as a regular subtraction problem

You are subtracting four of one thing from nine of it, even if one is four of x and the other is four ones, the actual number four. It's like doing math with strawberries like people do to explain it to little kids, but instead of strawberries, it's a placeholder for an unknown but constant value.