Arlinda
It depends how you want it to look after the repair. I have 2 sugesstions for you.
A. You can put a patch on it, which shows on the outside of the trousers, using a similar kind of material. This will show on the outside. It may not look nice, or you may like it.
B. You can first put a patch on the inside of the trousers, using any similar material. Then using zig-zag stitch (using the machine) carefully sew along the L shaped tear (sewing through the patch). This will also show on the outside, but not as much as A, if you use matching thread.
What the end product looks like will very much depend on you and your taste. Hope this solves your problem!
Shubido, November 2007