I assume that you want to solve cos(x) = 2*pi, not pie!
arccos(0.4) = 1.1593 radians.
This is the solution in the range 0 to pi. There is another solution which is at 2*pi - 1.1593 = 5.1239 radians.
Note that arccos appears on most calculators as "cos to the power -1".
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