(i) The driving force \(DF\) is given by \(DF = \frac{P}{18}\). Using the equation \(DF - R = ma\), where \(R = 800\) N and \(a = 0.5\) m s^-2, we have:

\(\frac{P}{18} - 800 = 1400 \times 0.5\)

\(\frac{P}{18} = 800 + 700\)

\(\frac{P}{18} = 1500\)

\(P = 27000\) W

(ii) At the new speed of 25 m s^-1, the driving force \(DF\) is \(\frac{P}{25}\). Using \(DF - R = ma\), we have:

\(\frac{27000}{25} - 800 = 1400a\)

\(1080 - 800 = 1400a\)

\(280 = 1400a\)

\(a = 0.2\) m s^-2