I have labeled the diagram here.
View attachment 296049
You are talking about
abcd.
It
could be a rectangle - though it sure doesn't look like one. It
could also be a rhombus, though that is unlikely too. A rhombus has four equal-length sides. We would need to know that
ab is the same length as
bc, and we have no reason to assume that.
All we really know is that it's a
parallelogram (because we
constructed it to have parallel sides).
But it's actually irrelevant which one it is; they all use the same formula: bxh.You cannot assume this. Though, again, its actual sub-shape doesn't come into play in this case.