Answer:
BD = 8 cm
Explanation:
Diagonals of trapezoid divides each other in equal ratio.
if ABCD is a trapezoid and the diagonals AC and BD intersect at point O
then we have

it is given that
and BO=6 cm
so we can write

cross multiply
divide both side by 3
OD= 2 cm
now we have
BD = BO +OC
BD = 6 cm + 2 cm
BD= 8 cm