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This problem is for advanced students (Math circle level or Math Olympiad level).
Therefore, I will be very short and will give the basic idea, only.
The given data allow to determine the side length of the given square.
How to do it, you will find in the lesson
- Find the side of a square if distances are given from an interior point to 3 its vertices
in this site, where similar problem was solved.
By knowing the square side length, you may apply then the Cosine Law and to find the angle ETF.
Happy learning and happy solving (!)