Answer:
150,000 Joules. 
Step-by-step explanation:
Given that a 50.0 kg shell is fired from a cannon on Earth's surface to a maximum height of 400 m. It then falls to a height of 100 m. What is the loss of potential energy as it falls?
From Potential energy formula, 
P.D = mgh
Assuming g = 10m/s^2 
P.D = 50 × 10 × 400
P.D = 200000 Joules 
Then it fell on 100m
The potential energy will be 
P.D = 50 × 10 × 100
P.D = 50000 Joules 
The lost Potential energy will be:
Lost P.D = 200000 - 50000
Lost P.D = 150000 Joules 
Therefore, the loss of potential energy as it falls will be 150,000 Joules.