Need with writing. i need to explain the purpose / main idea of the presentation. but i have no idea how to do that.

Just find what they have been saying a lot of times in the presentation, or described a lot of times, and that's your main idea, for example; what is the main idea in this question?
which animal jumps really high, there are many in the continent of Australia. They are the only animals that can jump that high. if a baby kangaroo jumps it's the average of we humans can jump. they are close to becoming extinct, because people are killing. share this message with everyone else, and save the kangarooos
Yes i know kangaroos do not have that many oo's but ooh well it looks good. :D hope you get the answer. :)

Wow this is hard hope you do good and get answrs