Hi qnguyen,

To embed a Khan Academy video in Google Slides, first go to "Insert" and then "Video":

After that, you can either type in "Khan Academy" followed by the topic, and get YouTube search results, or insert the video using the URL of the YouTube video this can be found from a Khan Academy video by clicking "Video on YouTube" after scrolling down underneath the transcript).

I don't believe it is currently possible to embed a video directly from the site onto Google Slides.

I hope this helps! Please let me know if you have any further questions!