Basically, the negative sign outside the brackets implies that it affects each term in the brackets.
3k – (2m + 3n – 5k)
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?