(i) Using the formula for power \(P = Fv\), where \(F\) is the force and \(v\) is the velocity. The force \(F\) can be found using Newton's second law \(F = ma\), where \(m = 80\) kg and \(a = 1.2\) m s\(^{-2}\).

Thus, \(F = 80 \times 1.2 = 96\) N.

Substitute into the power formula: \(P = 96 \times 5 = 480\) W.

(ii) The power \(P = 450\) W, speed \(v = 3.6\) m s\(^{-1}\), and \(\sin \alpha = 0.035\).

Using the formula \(\frac{P}{v} - mg \sin \alpha = ma\), where \(m = 80\) kg and \(g = 9.8\) m s\(^{-2}\).

\(\frac{450}{3.6} - 80 \times 9.8 \times 0.035 = 80a\)

\(125 - 27.44 = 80a\)

\(97.56 = 80a\)

\(a = \frac{97.56}{80} = 1.21\) m s\(^{-2}\).