If the function has a ∪ shape (opens up), then it only goes so low—has a minimum value.
Any even-degree function with a positive leading coefficient will open up, so have a minimum. Your function is degree 2 (even) and has a leading coefficient of 3 (positive).
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Comment on other polynomial functions
If the leading coefficient of an even-degree function is negative, it will have an inverted-U shape (∩), so will have a maximum.
Any odd-degree function will have a generally rising (/) or generally falling (\) shape, depending on the sign of its leading coefficient. If positive, then rising; if negative, then falling.