First, install the necessary packages
Xrandr

$ pacman -S xorg-xrandr

Connet to display with HDMI cable, show infromation.

$ xrandr

Display according to your environment

$ xrandr --output eDP1 --auto --output DP1 --auto --right-of eDP1

By changing right-of to left-of, the display can be changed to the left or right side.

If you wanna quite, use bellow command.

$ xrandr --output DP1 --off