"I am eating a good cake."
Translation:אני אוכל עוגה טובה.
If I were speaking to a man I would say “Ata tov” You are good or to a woman “At tova” You are good.
Similarly, if I were speaking about a man I would say “Hu tov” He is good or about a woman “Hi tova” She is good.
However, these scenarios are not what “I am eating a good cake” is about. Rutger correctly pointed out that tova, feminine “good” was used because the gender of the cake is feminine. 14 people agreed with him.
The gender of the adjective “good” has nothing to do with the gender of the person to whom you are speaking.
You have to take into account the gender of the person you are addressing when you are talking about what that person is doing. “You eat” Ata okhel to a male and At okhelet to a female.
The default sentence shown above is masculine. And when there are cases like this, the difference between the masculine and feminine version is just one letter ת, it usually just makes it a typo and doesn't reject the answer. That means that you must have had another mistake that you didn't see, if your answer was rejected.