A test vehicle moves along a straight track with its position given as a function of time by \(x(t) = (2.0 \text{ m/s}^3)t^3 - (6.0 \text{ m/s}^2)t^2\), where \(x\) is in meters and \(t\) is in seconds. What are the vehicle's instantaneous acceleration at \(t = 3.0 \text{ s}\) and its average acceleration over the time interval from \(t = 0\) to \(t = 3.0 \text{ s}\)?