Take a look at the number line show below:
You begin at #-8#. This is #8# integer units to the left of #0#. When subtracting a positive number from any number #n# (whether it is positive or negative) the value of that number #n# is reduced.
In this case, you are subtracting #2#. If you start at #-8# and count two integers to the left, you will arrive at #-10#.
The answer is not #-6#! For negative numbers, the larger the negative value, the smaller the number! This is different from positive numbers, where the smaller the positive value, the smaller the number. Subtraction by a positive number reduces the value of a number. So the number that is #2# smaller than #-8# is #-10#.