Can´t trim multiple surfaces

Hey folks, im a recent user of grasshopper, so i don´t know much about it. Pretty noob.

Im working on a lofted cover for lower limb protheisis, and i need to "trim" the front part of the cover and keep only the back, but i dont know how to do it. (ive tried many options, but none of the worked)

Can some one help me with it?

Thanks :)