Let's rewrite the polynomial in standard form with the leading term at the front.
Since the leading coefficient is less than zero, then the concavity of the polynomial will be decreasing around at least one of the endpoints. It doesn't matter how many terms we have; if the leading coefficient of a polynomial < 0, then we have a negative concavity.
Since we know the leading power is even, both end points will dig downwards, because for a polynomial with an even leading power, both end points follow the same way.
Thus, we know the endpoints will both dive down towards negative infinity, as x approaches infinity and negative infinity.