If you are working with the Cambridge "Prepare!" curriculum, several video tutorials cover the grammar and exercises found on page 77 across different levels:
For very large files (e.g., 77GB), users often use tools like Handbrake to compress the file into a more manageable MP4 format while maintaining quality. 77 (1)mp4
If "77" refers to a file size or a specific technical issue with an MP4 file, here are common solutions for preparing large video files for playback or sharing: If you are working with the Cambridge "Prepare