three of the six vertices of regular hexagon are chosen at random.the possibility that the triangle with three vertices is equilateral equals
three vertices can be chosen as 6C3 after that what to do

Hello, prasum!
I don't suppose you made a sketch . . .
Three of the six vertices of regular hexagon are chosen at random.
Find the probability that the three vertices form an equilateral triangle.
Three vertices can be chosen as 6C3. . Right!
After that, what to do?
. .
There are only two equilateral triangles: .and