Słubice is a little town at the German/Polish border. If you get dropped in Frankfurt (Oder), it might make sense to walk here (just over the bridge) to hitchhike into Poland.
Walk along the dike until the end of town. This road, national road 29, is the only route leaving the town to the south and goes straight to Wrocław. Several kilometers outside the city, there is an intersection with the E30/N2 which go east to Poznan and Warsaw.
West towards Germany
Instead of hitchhiking from Słubice, it is best to walk over the bridge to Frankfurt (Oder) and take a bus or tram to the German motorway.