Question 807298
 (18m/-4n)/(-1/4m) 
------
Invert the denominator and multiply::
-------------
= (18m/-4n)*(-4m/1)
--------
Cancel factors common to a numerator and a denominator::
---------
= (18m/n)(m/1)
-----
= 18m^2/n
=============
Cheers,
Stan H.