FCE Part 3 regularly tests words with prefixes. A negative prefix changes a word to its opposite; other prefixes change meaning. You must choose the right prefix.
un-
→
un- — opposite — unhappy, unemployed, uncertain
dis-
→
dis- — opposite — disadvantage, disagree, discrimination
mis-
→
mis- — wrongly — misunderstand, mislead, mismanage
over-
→
over- — too much — overestimate, overwork, overwhelm
under-
→
under- — not enough — underestimate, underfund, undermine
re-
→
re- — again — reconsider, retrain, reorganise