Hi Claudia,

I have found the Fill-in-the-blanks this relates to.

Normally you will need a definite article in French where you don't in English :

 

But in certain expressions you don't need the article as for instance  in the one you quote -

être bon/mauvais en = to be good /bad at 

 

The second example is different as it is justthe preposition 'de' ('of')separating the two nouns -

it could have been -

des heures de repos = hours of rest 

Hope this helps!