Subtracting One String from Another

I have two columns of data that represent days of the week. One column holds all the possible days of the week that a route can be run such as "MTRFS" or "MTWRF." The other column includes days that a route cannot run such as "TFS" or "M." I am trying to come up with another column that will subtract the cell from the days that a route cannot run column from a cell in the possible days column. Is it possible to subtract one string from another like this?

Shouldn't Days_Route_Can_Run - Days_Route_Cannot_Run = Days_Route_Can_Run? Seems like there should be no common day between when it can and when it cannot be run (i.e. mutually exclusive).

Pauley

Weekday Issue.xlsx

See above attachment for Input and Output examples. Yes the final column will be mutually exclusive. That's why I'm trying to do some sort of "String Subtraction." This is only my first thought. If anyone has any other suggestions then I'd greatly appreciate them

Thank you. This solution works perfectly.