"Thank you, Andrew and Isabel."
Translation:Tapadh leibh Anndra agus Iseabail.
Officially "leibh" is plural, but for more formal circumstances (like when speaking to a gradfather or a professor) you can treat a singular person as plural.
This used to happen in English too. We used to use "thou" when speaking to one person and "you" when speaking to multiple people. It became good usage to use the plural "you" to speak to a single person as a formal usage (like in Gaelic). Eventually, people just started to use the formal "you" for everyone and we lost the singular "thou" and now we use "you" for everyone singular or plural, formal or informal. But Gaelic still has the tradition of using the plural "you" to speak to a single person in a formal register.
If you are doing the course on iOS or Android, you cannot currently access the Tips & Notes through the app. To access the Tips & Notes, you will have to access the course using a web browser at https://www.duolingo.com/. You can still do it on your mobile device, but you will have to use the web browser instead of the app (or you can do it from a computer). When you click on a Skill, it will expand to reveal a Start button, a key, and a light bulb.
If you click on the light bulb it will reveal the Tips & Notes and give you a detailed explanation of the grammar that is introduced in that Skill. Duolingo is testing a new pattern where instead of a light bulb, there is a large button that says "Tips", so some people might see that instead.