hmm,
let's think, change 5760x1080 to 1920x1080.
5760 / 1920 = 3
1080 / 3 = 360
Number 3 is your scaling value, to keep the proportions (1920x360 is the resolution you should set).
You can not keep the proportions (nobody requires it), you fill with 5760x1920 content, 1920x1080 canvas, squeeze pixels in width (loss), and stretch in height (yes I know how it sounds, but that's how it will look).