Having an issue with choppiness

Back already lol.

 

I have a 30 second clip. 20 seconds of those were filmed on my Nikon P600. 10 seconds was filmed on a GoPro.

 

The first 20 seconds filmed with the Nikon are perfectly smooth when I am editing and I play it back AND also when I've exported the video. However, the GoPro part is very choppy while I am editing and also when I have exported the video.  This makes editing hard (trying to get it in sync with music), and very bad to watch the final copy.

 

I've looked stuff up about this but everyone's issue is that it's only choppy after exporting it, and the answers they receive are from someone who says things that the average person won't understand. I have no work experience in editing/software/videos. I need something easy to get and dumbed down. Can anyone help?

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