The negatives mean how much lower they are from the surface.
To find "how much lower," we need to subtract -23 from -187.
-187-(-23) is the same as -187+23 (2 negatives=1 positive)
-187+23= -164.
However, it's not negative in the answer because by asking how much lower, we don't need to have a negative. "How much lower" is a distance, and a distance is always positive.
This is because -10 x 5 = -50 and if you have 20 points to start with, you just do 20-(-30)=20 and two minuses make a plus so it is 20 + 30 which is 50.
