DE = 1.5 cm, EF = 2.25 cm, FD = 3 cm
Side DE corresponds to side AB and is (1.5 cm)/(2 cm) = 3/4 the length of it.
Because the triangles are similar, all pairs of corresponding sides have the same ratio. That is, the side lengths in ΔDEF are 3/4 of those in ΔABC.
... EF = (3/4)×BC = (3/4)×(3 cm)
... EF = 2.25 cm
... FD = (3/4)×CA = (3/4)×(4 cm)
... FD = 3 cm