How to play music on my iphone without it going away when I reopen a app?

I always open and app and it goes away

Open an app. Then double press the round home button and a multitasking bar should appear. Slide to right and your music controls should appear and you can tap the play button and enjoy your music. Just press the home button once to exit out of the multitasking bar to return to the app that you just opened. Hope this helps