The denominator is k, according to user who posted the question.

We need to solve for k.
In order to solve for k, we need to isolate k, meaning
1. subtract n on both sides in order to isolate k, simplify



2. multiply k on both sides, do not forget the parentheses,

3. divide by (m-n) (do not forget the parentheses), simplify


So the final answer is
k=1/(m-n) do not forget the parentheses.