Screen resolution problems

That is proprietary software, don't go there.

I think you might need a custom xorg.conf (man xorg.conf). Or you could try a newer Linux-libre kernel from http://www.fsfla.org/ikiwiki/selibre/linux-libre/#downloads

Trisquel is commited to FSF's Guidelines for Free System Distributions: https://www.gnu.org/philosophy/free-system-distribution-guidelines.html

Nobody here will help you with non-free software. You could try a newer Linux-libre kernel, as lembas suggested; I suggest http://jxself.org/linux-libre/ or https://launchpad.net/~linux-libre .

Have you tried selecting your resolution using the Display control in the System Settings?

If that doesn't work, you could try installing the Resolution Switcher application. Its in Add/Remove Applications, search for "resolution".

The Resolution Switcher can have more resolution options than the one in System Settings.

You should uninstall this proprietary software and probably redo your entire Trisquel install to get rid of any components that are leftover by you installing the proprietary driver.

Trisquel is a 100% Free Software distribution. In order to use it, you must dedicate yourself to using only free software (see https://www.gnu.org/philosophy/free-sw.html). If you absolutely must use the proprietary nVidia drivers (hint: you don't, there exists a free software nVidia driver that supports most (if not all) nVidia chipsets already included with Trisquel 6), don't use Trisquel or any other free software operating system. Nobody here is going to help you use non-free software, as it is against the reason that Trisquel was made.

If software isn't included in Trisquel's official repositories, there's probably a reason why it isn't.