There is no such thing as ‘an optimal feasible area’.
The feasible area is the area where all of the constraints are satisfied (i.e. the area ‘below’ all the constraint lines.)
The optimal mix is at the corner of the feasible area that is furthest away from the origin when moving out the contribution line while keeping it parallel.
Have you watched my free lecture on this, because I do make the approach very clear in the lecture.