Just subtract each power of m and n, subtract the bottom from the top. You can not do this to 2^3 and 4^2 because they had different bases. Expanding, you get that 2^3=8 and 4^2=16.
Therefore, the numbers cancel out, leaving a 2 in the denominator.
For m, 4-5=-1, which means the variable m is on the denominator.
For n, 3/2-1/2=1, which means it is on the numerator.
So, simplifying this expression gives you n/2m.