1. Acer Aspire 5742 Ubuntu Brightness Fix.
  2.  
  3. Open a terminal window.
  4. Type "sudo gedit /etc/default/grub" without the quotation marks.
  5. Look in the text file for GRUB_CMDLINE_LINUX_DEFAULT=" (May contain extra text following)
  6.  
  7. paste
  8. GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux
  9. over it. and hit save. close the window.
  10. In the terminal window, now type "sudo update-grub" without the quotation marks, restart your system, and now with the keyboard hotkeys you can control brightness.
  11.  
  12. For any further help, email me at Liamlemage@gmail.com