A rectangular solid measures 1.0m by 5.6cm by 2.1mm express its volume in cubic meters

First, make sure all three measurements have the same unit (I recommend meters because that's what the question calls for)

1.0m=1.0m

5.6cm=0.056m

2.1mm=0.0021m

Then, you just want to multiply the three together to get the total volume.

1.0X0.056X0.0021=0.0001176 or 0.00012, if your teacher wants you to use proper sig figs