work at

Từ điển WordNet v3.1 - WordNet Dictionary

verb

to exert effort in order to do, make, or perform something (Freq. 6)
- the child worked at the multiplication table until she had it down cold
• Syn:
work on
• Hypernyms:
work
• Hyponyms:
belabor, belabour
• Verb Frames:
- Somebody ----s something