Sounds decidedly home-work-ish to me, but what the hey...
/If/ B.display() shadows A.display(), i.e. there's no overriding
involved - my C++ is a /little/ rusty ;-) - then you can do this
Simply tell ("up-cast") 'B' to behave as if it were an 'A'.
However, if B.display() /overrides/ A.display() - don't think this is
the case, but just for completeness - then you're stuck.
Unless you expose an alternative method on 'B' that calls the base
implementation in 'A', then you can't access the base implementation in
A; and that's as it should be.