I don´t know how to solve this with my calculator.

I know that a piece of arc is 1/4 of the Circle´s circumference and that the circumference of the circle above is 6pi

It says like this in my book:

Arc MN = 40/360*

And solve 360(ARc MN) = 240 pi

Arc MN = 2pie/3

My calculator is a TI-82 Stats from Texas Instruments the blue one: http://www.trion.si/ArtikelSlike/004253.jpg