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Comparing with your starting post
- https://www.algebra.com/algebra/homework/Finance/Finance.faq.question.1115623.html
you made a huge progress. My congratulations !
But you incorrectly wrote these constraints x >= 0, y >= 0.
Actually, they are 20 <= x <= 60 and 20 <= y <= 60 and (despite to @greenestamps statement),
these constraints make influence to the feasibility domain: this domain is bounded (is entirely inside)
the rectangle 20 <= x <= 60 and 20 <= y <= 60 shown in the figure below:
So (again), your feasibility domain is the 5-sided (=5-vertices) polygon concluded inside the rectangle and located below the two sloped lines (red and green).
Thus you must calculate and compare the profit function in all 5 vertices of the polygon.
Good luck !