Acer Aspire 5742 Ubuntu Brightness Fix.
Open a terminal window.
Type "sudo gedit /etc/default/grub" without the quotation marks.
Look in the text file for GRUB_CMDLINE_LINUX_DEFAULT=" (May contain extra text following)
paste
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_osi=Linux
over it. and hit save. close the window.
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.
For any further help, email me at Liamlemage@gmail.com