The problem is the nature of x+2/x-3 when x is a range variable. It is a list of values, not a single value, nor a range variable, nor even a vector (although it can be displayed like a vector). A list of values cannot be assigned to a variable.
The rule that Mathcad uses to prevent this is that if a range variable appears in the expression to the right of :=, it must appear to the left. So you could write y_{x-4}:=x+2/x-3. Now you have a list of variables on the left, and it can assign each of the values of the expression on the right to one of the variables on the left. That creates a vector out of the list of values, and you can then use normal vector concepts in handling the result.
Tom Gutman