Translation:My grandma is seventy-five years old.
This is the major flaw with Duolingo. There was no Tips option at the start of this lesson. You're just thrown in with no explanations, and have to get things wrong, forcing you to go to discussions in hopes someone who knows the language will volunteer a fuller explanation. I'm really starting to dislike Duolingo's approach.
Yeah, I know your frustration. The approach is natural acquisition - how you learned your first language. You made lots of mistakes, but kept hearing it over and over and finally got it right. When we were 1 and 2 we didn't care if we were wrong. We were happy each time we got our idea across. With full language on board, we get frustrated with the new language. Do take time to write everything down as you go. It speeds up acquisition, and you will find the patterns, just like with your first language you found the patterns. We will always encounter new words from time to time, if we keep reading and talking to people. It's a bummer to lose a heart this way - but I promise you the shock of getting this wrong seared it in my memory and I have never missed it again.
Remember that if you are on a free online resource, from a business perspective, you are the product, not the software. I don't assume that anything that is on here is correct, or that what is presented is done with the user's benefit as the primary consideration. This question is a perfect example because although the given answer is correct, there are other correct answers that are marked wrong. Nothing about the word tūtū implies a gender of the grandparent being spoken about. Below is a very good free online resource book. Two of the authors lead the regular sunday DL Hawaiian meetup. As far as I can tell they are interested in Hawaiian, and being teachers, organized the grammar patterns from many different Hawaiian grammar books for their own personal study, and generously shared their work with the rest of the world for free.