Answer:
-252.87 °C
Step-by-step explanation:
The problem statement tells us ...
boiling point - melting point = 6.47 °C
boiling point - (-259.34 °C) = 6.47 °C . . . . . fill in given information
boiling point = 6.47 °C - 259.34 °C . . . . . . subtract -259.34 °C
boiling point = -252.87 °C