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Through Which Free App On Play Store?

Not on play store..not at least free.. Search them on blackmart or Google them.. Here are the two apps I have used for merging Movie editor Vid trim/pro video editor

Compress PDF: All You Need to Know

It's the same basic app, the only difference is I changed the app.   You could also try M4A, but I haven't tested it yet. What I suggest to you is make some kind of simple video file editor, so you will know what you have to edit. Don't be afraid to try stuff.