Question 1124891
Temperature change = (final reading) - (initial reading) = {{{ -9^o - 15^o = highlight( -24^o ) }}}<br>

The minus sign tells you there was a decrease in temperature.   If you want (or if you are required) to specify the change as a positive number then it is acceptable to say  "the temperature decreased by 24 degrees."<br> 

( It is analogous to calculating the net profit for a company:  when the calculation comes out negative, one might prefer to say the company lost X dollars, rather than  "the company made -X dollars."  )