a) 

8cmx3,5cmx2cm

$V=8cmx3,5cmx2cm=56c{m}^3$

b)

7dmx7dmx7dm

$V=7dmx7dmx7dm=343d{m}^3$

c)2,5dmx50mmx40cm

25cm x5cmx40cm

$V=25cmx5cmx40cm=5000c{m}^3$