Explanation:
Given parameters: 
 Temperature at 8pm = 6°C
 Temperature at 2am = decreased by 13°C
Unknown: 
Temperature at 2am
Solution: 
Since this is a temperature decrease problem, the temperature at 2am should be lesser. 
 Temperature at 2am = Temperature at 8pm - Temperature decrease
 = 6°C - 13°C
 = -7°C