When there is a negative sign in front of a number it means the "opposite" of that number.
So for instance -5 or negative five is the "opposite" of 5 which is -5.
So if you have a number like -3 what would it it's "opposite"? That would be 3.
so if you have two negatives its like taking the opposite of an opposite. so the opposite of -7 is 7.
Which is why in this case you would get 17 + 7 = 24.
Khan academy has a few videos on this which goes into much better explanation also has tons of information that should be able to help you with some mathematics.
https://www.khanacademy.org/math/cc-sixth-grade-math/cc-6th-negative-number-topic/cc-6th-number-opposites/v/opposite-of-a-number