"An t-amar agam."
I have the bathtub would be tha an t-amar agam. In this example, an t-amar agam, there is no verb here, it is just my bathtub; the literal meaning is the bathtub at me and you might think of it as a similar construction to English the bathtub of mine.
But in Gaelic this is used much more often than in English, as pretty much always when you deal with alienable possession. You might want to read the tips and notes to the Body 2 skill and two articles about possession on the Akerbeltz wiki: Possessives and syllabic structure or Ar n-Athair a tha air nèamh and Aig, air agus ann an or The severed head.
You can read the tips and notes to the lessons in the web browser version of Duolingo at https://duolingo.com, and also on the https://duome.eu/tips/en/gd website. Unfortunately they might not be available in the Duolingo mobile app – so if you use Duolingo on a mobile device, you might want to open your browser and head to duolingo.com for your reading.