Prime Implicants in Boolean Function

Question

A Boolean function $F(X_1, X_2, X_3, X_4, X_5, X_6)$ of six variables is defined as $F = 1$, when three or more input variables are at logic 1. otherwise 0. How many essential prime implicants does F have?

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How to visualize this problem. A Six Variable k-map is very difficult to manage. Any hints?