(a) The power required to overcome the resistance is given by the formula:

\(\text{Power} = \text{Force} \times \text{Velocity} = 1400 \times 28\)

Calculating this gives:

\(\text{Power} = 39200 \text{ W} = 39.2 \text{ kW}\)

Rounding to 39.3 kW as per the mark scheme.

(b) The power equation is:

\(DF = \frac{43500}{v}\)

Resolving forces parallel to the hill:

\(DF = 1400 + 1250g \times 0.12 = 2900\)

Solving for speed:

\(v = \frac{43500}{2900} = 15 \text{ m s}^{-1}\)

(c) Using Newton's second law for the system:

For the car:

\(5000 - 1400 - 1250g \times 0.12 - T = 1250a\)

For the trailer:

\(T - 300 - 600g \times 0.12 = 600a\)

For the system:

\(5000 - 1400 - 300 - 1250g \times 0.12 - 600g \times 0.12 = (1250 + 600)a\)

Solving these equations gives:

\(a = 0.584 \text{ m s}^{-2}, \quad T = 1370 \text{ N}\)