Sometimes my Acer would boot into Ubuntu 12.04 without a back light and I cannot adjust the screen's brightness

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http://askubuntu.com – I have an Acer 4736z and I have installed Ubuntu alongside Windows 7. Ubuntu works great but sometimes my computer does not have backlight when opening Ubuntu and I have to reboot the PC several times until I get a backlight. This has also happened on Ubuntu 11.10. And another thing, I cannot adjust the screen's brightness. Please tell me what to do to resolve this. Thanks. (HowTos)