"I know these cities."
Translation:Я знаю эти города.
I got it wrong as well :), it's just an irregular form, the most common ones are given on this page : http://www.russianforeveryone.com/Rufe/Lessons/Course1/Grammar/GramUnit2/GramUnit2_2.htm
Note that the linked article refers to a stressed ending. That changes the pronunciation of города:
Genitive (stressed 1st syllable): "Gó-ra-da"
Nominative/accuative plural (stressed ending): "Ga-ra-Dá"
The change is due to the difference between stressed and unstressed "o" in Russian.