Think about this.......If the temperature was -8 and then it dropped another 7 degees....would it have risen 15 degrees?? No.....it would have dropped -8 + -7 = -15 degrees !!
You're getting this confused with a negative times a negative, which = a positive.
This one is harder to see......suppose you have lost 7 dollars for the last 8 weeks......then in 8 more weeks, you will have 56 dollars less...(8) times (-7). But 8 weeks ago, (-8), you had (-8) times (-7) = 56 dollars more. Does that make sense??
So, adding two negatives produces a negative, but multiplying two negatives produces a positive.
Hope that helps......