A catalyst increases the rate of reaction by providing an alternative reaction pathway with a lower activation energy. The activation energy of the uncatalysed reaction is +250 kJ mol
-1. So the activation energy of the catalysed reaction must be less than that, and because energy is needed to be supplied in order to break bonds, the activation energy must be positive (greater than 0).
For part vi) you're going to want to do something with the E
a for the forward reaction, and the ΔH value for the forward reaction. Hint: as you're looking at the reverse reaction, you'll want to "reverse" the -92 kJ mol
-1