What is your exact location if you are crossing the prime meridian and the altitude of Polaris is 50

Crossing the prime meridian it means that you are at 0 longitude. Polaris, the star, is right above the north axial pole, in other words, 90º north. So, if Polaris is of altitude 50 then it means you are at 50º north. The exact location would be 50ºN and 0ºW.