Thank you all! Building upon the logistic regression solution, I have noticed that Lin[Fail] function is positive for [Fail] category and negative for [Pass] category. So, it is intuitive to suggest that Lin[Fail] is zero on the boundary between the predicted [Pass] and [Fail] categories. Since the formula for Lin[Fail] is an output from the model, equating Lin[Fail] to zero in this formula gives the boundary equation I am after.

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