The question was not only to you.
The capital region of 10th century Poland was around Poznan - on Warta, but above Notec confluence - and Gniezno.
The capital of Masovia was not yet at Warszaw - it was at Plock. On Visla - and below the Bug confluence.
Where was the border between Kiev state and Poznan state in 10th century?
"Kiev state" as a single meaningful entity did not exist. Term "Kievan Rus" is applicable to a collection of the princedoms ruled by the members of Rurikid dynasty with the succession being passed (in theory) to the most senior member of the family (with the resulting domino effect on some other princedoms due to the shifts of the junior members of the family to the vacant throne(s) according to their seniority within family. Of course, being relatives did not mean being friends and all these princes routinely fought each other. Even accession to the most senior throne of Kiev was as often as not decided by force and none of its rulers (AFAIK) had unquestionable control over the whole "Rurikid Empire". IIRC, at least some of the "borderline" princedoms were not even necessarily ruled by the Rurikids.