Do you mean g(x) = -3x^3 - 2x?
Do you see that the biggest exponent is a positive odd number 3? See it?
This function is an odd-degree polynomial. However, since the leading coefficient of this odd-degree polynomial is negative (the leading coefficient is -3), then its end-behavior represents BOTH ENDS of the graph going in a downward direction.
Think of a giant letter M to get an idea.