(a) Byli jsme na návštěvě u Čechova. Cimrman Čechovovi pohnojil višňový sad.

First step: identify the common entity: Čechov

Second step: který must take the gender and number of Čechov: masc. sg.

Third step: který must take the case from the second clause: Čechov is in the dative (indiret object of pohnojil)