teškoća

1. difficulty; problem; hardship
   Firma ima finansijske teškoće. — The company is having financial difficulties.
   Sa teškoćom je ustao iz kreveta. — He got out of bed with difficulty.

In the sense "problems" it is often plural: imati teškoća / teškoće sa + instr. For the difficulty level of a task or question Serbian usually uses težina (težina zadatka), not teškoća.