I don't get these, and these questions always pop up:

Here is an example:

Below are some expressions:

?ab

a + 2b

?aČ + b

The letters a and b represent lengths

? and 2 are numbers that have no dimensions

State whether each Expression is a Length, Volume, Area or None of These.

I don't get how you do this.. What does it mean they have no dimensions, and what would each expression be?

Thanks in advance