Question 572516
if i understand your question correctly, you are looking for the dimensions of the trapezoid, the upper half of which will be the base of the triangle on top.
the best way to show you what i mean is by the diagram below.
it has the measurements you are looking for i believe.
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ABC is your original triangle.
ADE is the top part of the coop.
DECB is the bottom part of the coop.
DE is parallel to BC so it forms triangle ADE similar to ABC because all the corresponding angles are equal.
if the triangles are similar, then their corresponding sides are proportional.
i drew EF parallel to DB to show you that all 3 triangles are similar.
those triangle are:
ADE, ABC, and EFC
your coop can be constructed with or without EF as a brace.