Adding/Subtracting in binary and hexadecimal formats?
M - N can be done in the usual way as:

  00111100110100.1011 -  
  10010001111101.1010 
---------------------
1 10101010110111.0001
---------------------


We have a borrow here. Answer is negative and is in 2's complement form.

Or you can add the 2's complement of N with M.

-N = 2's complement of N = 101101110000010.0110

 0 00111100110100.1011 +  
 1 01101110000010.0110 
  ---------------------
 1 10101010110111.0001
  ---------------------


Take the 2's complement of this number to get -01010101001000.1111 = -5448.9375.