Can you say "Ce vei rezolva cu acesta"? It marked me as wrong and corrected it to aceasta.
Yes, you can. As a matter of fact, your solution is the literary one, Duo's being just a colloquial expression, used at street corners.
Years later, vei rezolva is still not accepted. This form (voi / vei /vor etc) is accepted throughout the rest of the exercise. This is an error that should be corrected, Duo!