Then AB = 3X. DC = 5X。

Because E. F is the midpoint of ab and CD, respectively.

So EB = 1/2 AB = 1/2 × 3X.

DF = 1/2DC = 1/2 × 5X

EF = ED + DF

= ( EB - DB ) + DF

= ( 1/2 × 3X - X ) + 1/2 × 5X

= 3 times

Because EF = 6cm

So 3X= 6.

So X = 2.

So AB = 3X = 6cm.

CD = 5X = 10cm。