Energy = mass ×specific heat capacity×change in temperature'
In this case, the mass of water is 50 kg, the specific heat capacity of water is 4184J/kg
the change in temperature is 80-45= 35 degrees celcius
Therefore, heat energy
= 50 ×35 ×4184=7322000 Joules
But 1kJ =1000 Joules
therefore, the energy needed will be 7322 kJ