Mark M. answered 03/11/21
Retired math prof. Calc 1, 2 and AP Calculus tutoring experience.
f(x) = cotx = cosx / sinx
sinx = 0 at x = π, 2π, 3π
So, f(x) is not defined at those points.
Therefore, f(x) is not continuous on the interval [π, 3π], so Rolle's Theorem is not applicable.