I would do this with Taper. Call Taper. In your right viewport, with Project on (on the OSnap toolbar), select the bottom of the shape as the start of taper axis, the top as the end of the taper axis, then for what you are looking for be sure that Flat=Yes and Infinite=Yes.
Thank you pascal. You are the greatest teacher there is.
Though i am novice i would also like to be a regular and help other people starting out with rhino. But most questions seem to be advanced. is there a ‘novice’ tag where beginners post their questions. thanks again
Ankit, We were all novice at one time.
Welcome to the forum you may find that in the future you have learned something on your own that answers a question someone asks.
Keep checking daily it is a great way to learn.
There is another way to do this that will produce a cleaner, simpler and more accurate result.
All you have to do is untrim the planar sides and rotate them inwards and then trim and join. modelx.3dm (94.1 KB)
jim’s method also has the benefit that, by moving the planes, one can adjust the width and angle of the thing in future design iterations; overbuilding always has its benefits down the road ; )