I don't get this... the hint said "Umkreis" means "surrounding area" so I entered that, but that was wrong. It says the answer is "perimeter", but in the previous lesson I learned Perimeter = Perimeter in german?
I guess, here 'surrounding area' would be the most natural translation. The word 'Perimeter' is rare in German. For the perimeter of a geometric figure, we usually use 'der Umfang'. Note that in a mathematical context 'Umkreis' can also mean 'circumscribed circle'.