Split mesh

Hi all,

I'm quite new with grasshopper and I think there is an easy solution for this, however I can't solve it. I have a mesh which I want to split along four lines and remove the triangels (see picture). I tried to use the component mesh split plane. To use this component I made a plane in z-direction from the line where I want to split the mesh, but the component doesn't work. Does someone knows what I am doing wrong and how I can solve this?

Wessel