Decline 'exercitus, exercitūs' (m.) — army — in all cases (4th declension).
Hint: Watch the long ū in most endings and -uum in genitive plural.
Decline 'manus, manūs' (f.) — hand — in all cases (4th declension feminine).
Hint: manus is one of the few 4th declension feminine nouns (along with domus).