Any two negative numbers will always give you a positive number, so let's just ignore the - for now.
Straight away, two numbers that multiply to give 72 is 8 and 9. This is just basic multiplying. Now, to check if it adds to 17 (because we're ignoring the minus for now), you just do 8 + 9, which coincidentally happens to be 17. So you just place a minus in front of 8, and a minus in front of 9, and you get -8 × -9 = 72, and -8 + -9 = -17.
