How do I find the floor of controlled airspace on sectional charts?
It does not say it anywhere because the answer is not variable like class B airspace but lets dig in. 

The question here is really trying to get you to identify the airspace that surround Coeur D'Alene Airport and we can see the dashed magenta line encircling it. If you take a look at the legend you will see that denotes 


  Class E (sfc) Airspace.


The same goes for Sandpoint Airport which is surrounded by a faded magenta line. The legend again denotes 


  Class E Airspace with a floor 700ft. above the surface