As, triangle ABC and DEF r similar ..... and DE = 2AB and BC = 6 cm .........
then according to sides theorem :-
AB / DE = BC / EF
----> AB / 2AB = BC / EF [as DE = 2AB]
----> 1 / 2 = BC / EF
----> EF = 2BC
----> EF = 2 * 6 = 12 cm .........(ans) ...