πραγματεύσασθαι [1 verse](verb aor inf mp) "Occupy" is pragmateuomai, which means to "busy oneself", "take trouble", "to be engaged in business", "spend one's time in business", "take in hand", "treat laboriously", "be engaged in", "take in hand", "treat laboriously", and "be engaged in." -- "Occupy" is a verb used uniquely by Jesus here that  means to "busy oneself", "take trouble", "to be engaged in business", "spend one's time in business", "take in hand", "treat laboriously", "be engaged in", "take in hand", and "treat laboriously", The form is not a common. This is an infinitive "to busy themselves" or "to do business for yourself". This seems to be part of the narrative, the infinitive pairing with the "he said...to busy themselves".