Ah it si difficult to explain here,But I will give you the formulae.Try to remember them.

When equal resistors R are arranged in the form of a cube,then

(i)Eq.Resistance across an edge=7R/12.

(ii)Eq.Resistance across a face diagonal=3R/4.

(iii)Eq.Resistance across a body diagonal=5R/6.

Here,It is case(i).So,the ans is 7R/12.Hope this helped u.