all i get if I shorten the music to the same length as the video - the music speeds up which is not what i want so how do I slow it down and stop it when my video has ended please
Not sure what you mean. If you mouse over the vertical edge of the waveform at the end of the song, your mouse will change into 2 headed arrow and you can click and change the length of the song. It should cut off ( unless you fade it) and not speed up to finish the song.
What I do when faced with this situation is to simply cut the music track at the point where the video ends and then create a longish (perhaps 2 or 3 second) fade out of the audio track so that the music and the video stop at the same time.
You should see your audio track with the volume line somewhere in the center along the track, mouse over that volume track. You should see that 3 handles on the volume line - the center is the volume handle, and the 2 circular handle at each edge. Grab one of the circular handle and move it toward the center, you will see the diagonal line "fade" towards the end. That will fade out the song.