Flatening an area between two edges

Hi there blenderers, a beginner question here, how do i flaten an area between two edges without doing it manually edge by edge ? see image attached. many thanks in advance !

Duplicate the start and end edges, move them off to the side, and separate them into a new object. Bridge the edges so there’s a single face in between. With the face selected, create a new custom transform orientation in the top menu and use it to scale the faces on the original object to 0.

Select the two outer polys, use numpad shift+7, or shift+1 so you are seeing them “side on”. set your orientation to view, then scale to the appropriate view axis. The easiest way is to turn on the scale gizmo so you can see which it is. Then just scale, for example y axis to zero.

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Thanks for your replies, the video helps a lot ! will follow it.

You can also enable loop tool addon included in blender select your polys and in loop tools press flatten. W key menu.