That's the hardest part, because it doesn't work as usual. There are always exceptions.
The normal rule is if a word ends in अ as in लड़का , then it's masculine otherwise if it ends in इ, ई as in बिल्ली then it's feminine.
I wasn't sure if it was ki or kā so I just guessed. It said I got it right, but it also showed the right answer being the opposite of the one I guessed. Does that mean that in some cases, the 's is interchangable?