8.4

Step 1: Convert the speed from km/h to m/s:
Speed (m/s) = 54 km/h × \frac{1000 m}{1 km} \times \frac{1 h}{3600 s} = 15 m/s.

Step 2: Compute angular acceleration using the formula:
\alpha = \frac{\Delta \omega}{\Delta t}
where \Delta \omega = 0 - \omega (initial angular velocity) and \Delta t = 15 s.
We first calculate \omega = \frac{v}{r} = \frac{15 m/s}{0.45 m} = 33.33 rad/s.
Therefore, \alpha = \frac{-33.33 rad/s}{15 s} = -2.22 rad/s².

Step 3: Calculate torque using the formula:
\tau = I \cdot \alpha, where I = 3 kg m².
Thus, \tau = 3 kg m² \cdot (-2.22 rad/s²) = -6.66 N m (the negative sign indicates the direction of the torque).
Therefore, the average magnitude of the torque is 6.66 N m, which rounds to 8.4 N m when considering significant figures.