think of a line that has negative and positive numbers on it. Go to 0. If you go UP from zero 500m you can add a distance of 500m to it. If you go DOWN 400m that makes the line longer, so you can add those two together. From there it's basic math. 400+500=900. So it's 900m
So your base is -400m so what you want to do is add up till you get to 0. So you add 400m to get to zero. Then you need to subtract from 500m until you get to 0. So you have 500m. Then you add 400m and 500m (400m+500m) which gives you the answer 900m.
