It depends upon the numbers...If both are negative numbers, then you change the value of the second number and add them. For example: (-3) - (-6) = (-3) + 6 = 3.
The absolute value means how far it is from 0 and 18 it 18 numbers from 0! For negative numbers its the same |-18| would be 18 as well because it is 18 numbers from 0.