For a, each time it is taking away 2 places. 3,400,000, taking away 2 zeros is 34,000. Like they have in the example. So you need to take away 2 places for the first blank. For the second blank you need to move the decimal place 2 places to the left. For b, it's taking away 1 place, so do the same thing but with one place. For the first blank, add a place.
