what is this anwser to this question? 

      -54 decreased by the difference between -37 and 15

Replace the words "the difference between -37 and 15" by 
"-37 MINUS 15"

      -54 decreased by -37 MINUS 15

Replace -37 MINUS 15 by -37 - 15

      -54 decreased by -37 - 15.

Replace -37 - 15 by -52

      -54 decreased by -52

Replace that by

      -54 MINUS -52

Replace that by

      -54 - (-52)

Replace that by

      -54 + 52

Replace that by

      -2. 

Edwin