Basically, the negative sign outside the brackets implies that it affects each term in the brackets.

3k – (2m + 3n – 5k)

becomes

3k - 2m - 3n - (-5k)

3k - 2m - 3n + 5k

3k + 5K - 3n -2m

8k -3n - 2m

Its as though you had just the expression

-(2m + 3n – 5k), the negative representing a -1. You would simply expand out the brackets, yes? to make it -2m-3n+5k. Does this make sense?