External monitor resolution it's too low in Ubuntu 12.04

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http://askubuntu.com – I'm trying to use a 21.5 inches monitor (Síragon) as a secondary display for my laptop (HP Pavilion dm4-1265dx) but Ubuntu says the external monitor is "unknown" and don't let me increase the resolution above 1024x768. Can somebody help me out please? (HowTos)