Open nvidia control panel
Manage 3d settings
Program settings
Find arma somewhere
Select to launch with 960.
Also onboard graphics are your intel igpu. It isnt used for gaming and mainly for running displays iirc.
Or ArmA is stuff and doesnt support your display cable or whatever. For example Lakka doesnt want you to use HDMI for some reason so you have to use VGA wich is older then my grandmother. Honestly who still uses VGA?