launchBubblePosition variable in Frozen Bubble

Hi all
Can anyone explain launchBubblePosition variable in FrozenGame.java in Frozen Bubble Game
And when i touch on screen launchBubblePosition = (Math.PI - Math.atan2(yy, xx)) * 40.0 / Math.PI (xx = touch_x - 318, yy = 406 - touch_y) (FrozenGame.java file line 761), what’s this line mean? I don’t understand that code what doing.
Many thank!

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