Ok, here's a trick: If you're converting a smaller unit to a bigger unit in metric system, you divide by 10. When you are trying to convert a bigger unit to a smaller unit, multiply by 10. Decimeters are 1/10 of a meter, so you are going to multiply 2 by 10 because you are converting a bigger unit to a smaller unit, in which you get 20.
So your answer is 20 decimeters.