Miller indices are the reciprocals of the intercepts, made by the plane on the crystallographic axes, when reduced to smallest integers.

Miller indices are used to designate the planes in a crystal lattice.

The intercepts are OA, OB, OC

i.e. 3, 2, 1

The reciprocals of intercepts are 1/3, 1/2, 1/1

Taking LCM we get 2, 3, 6

So the miller indices of plane ABC are (2,3,6)