what I see right away is the equality of the 2 digit positions between -165 and +65. I can immediately combine them to -100.
and then the rest is easy : 3217 - 100 = 3117.
173 + (-43) + (-97)
I see that -43 and -97 combine nicely too: I can take the -3 of -43 and add this to -97, which gives me -100, while it leaves -40 on the former -43 side. it is now easy to combine these to -140.
