First of all thanks everyone so much for your attention.
The method i have to use is indeed the one BOB linked to (the one at WIKIPEDIA - i am not allowed to post links!).
So, I can use it for a system of linears, with ease, but i have no idea how to apply it on a system like the one i presented!!
Maybe some trick is required!?
I would be so gratefull, if someone has a hint!
this forum seems so nice!