Power plant cost accounting
1 kWh is 1 kW (power) for 1 hour, 1 hour is 3600 s, so 1 kWh is 3.6 MJ, but you don't need this as you are not asked to calculate the efficiency (Which is clearly poor).

The question is mostly a nasty excersize in units conversion, or in noticing the shortcuts. 
For example you need 14.5 MJ per kWh of electrical output, so at 300 MW electrical, you need 300,000 kW * 14.5 MJ/kWh of heat energy for 1 hour of operation, thus you can figure how many kg of coal or m^3 of gas will be burned in one hour, hence the prices. (Mind the units coal and gas are priced in).