21:9, 2560x1080 with AMD card

You can create mode resolution using xrandr and gtf

First use gtf to create a mode line

gtf 2560 1080 60 

this will generate something like this

# 2560x1080 @ 60.00 Hz (GTF) hsync: 67.08 kHz; pclk: 230.76 MHz
  Modeline "2560x1080_60.00"  230.76  2560 2728 3000 3440  1080 1081 1084 1118  -HSync +Vsync

then create a new mode using xrandr and parameters from previous command

 xrandr --newmode "2560x1080_60" 230.76  2560 2728 3000 3440  1080 1081 1084 1118  -HSync +Vsync

Now you need to add the mode to the desired output, in your case it seems to be HDMI-0

xrandr --addmode HDMI-0 2560x1080_60

Finally apply the new mode

xrandr --output HDMI-0 --mode 2560x1080_60

Cheers