Ex-interest is the MV assuming that this years interest has just been paid (as opposed to cum interest which would mean that this years interest was about to be paid).
I have explained in my reply to your previous question why we do not use the after-tax cost of debt, which has nothing at all to do with the question asking for an ex-interest value.
You really should not waste time attempting questions in the Revision Kit until you have fully studied everything in the syllabus.