Here is a question I’ve seen on Reddit 
OpenMandriva Lx 3.02 / Plasma 5 / X86_64
https://www.reddit.com/r/OpenMandriva/comments/7979hy/where_is_the_brightness_control_for_laptops/
I’ve seen this problem in a DELL computer. However, this problem is reported in other OS’s like Windows, Mac and other Linuxes.
For instance,
drivers - Brightness control not working on laptop ubuntu16.10 - Ask Ubuntu
Here, a LG laptop, brightness keys works fine with OMV LX 3.0.
There are applications (like xbacklight) that may be of some help. I have not tested since I don’t need it
Fn>F2 on my notebook would turn wifi on and off. Fn>F3 does nothing. Fn>F6 increases screen brightness and Fn>F5 decreases.
Things like this would be influenced by graphics hardware and driver which is not stated. As always it is wise to be sure user has an up to date system as well.
I can’t post on reddit. I guess that is some kind of social media thingy. I’m far to anti-social for that. 
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