Answer :

Multiples of -140:
1 and -140 is wrong because 1 + -140 = -139
-1 and 140 is wrong because -1 + 140 = 139
2 and -70 is wrong because 2 + -70 = -68
-2 and 70 is wrong because -2 + 70 = 68
4 and -35 is right because 4 + -35 = -31
Answer: 4 and -35

The multiples are -35 and positive 4.
First, you have to find all factors starting at 2, that can fit into 140. Take each of those numbers and find the other factor. 
When you get to 4, you see that -35 times 4 is -140, and also add up to -31.