In some ways they are both adding. If you are subtracting a number, what you are really doing is adding a negative.
3-2=1 which is the same as 3+(-2)=1
Once you know this you can use the fact that for addition it doesn't matter which order the numbers are in (just remember to carry the sign over with you)
6-10 means the same as 6+(-10) so I can rewrite it as -10+6 so I am stating at -10 and moving 6 places on the number line to the right (addition always moves to the right) so I get to -4
You can think of it as losses (negative) and gains (positives) if you add a loss to a loss you have a greater loss so the answer will be a smaller number than the ones you started with if you add a gain to a loss you have more than you started with.