Well, first of all, you have to open up the bracket by multiplying what's outside by what's inside. Anyway;
3k-15=-16
Next, you have to collect the like terms and put them together. And the -15 becomes +15. So;
3k=-16+15
3k=-1
Next, you have to divide the both sides by 3. So;
3k/3=-1/3
k=-1/3
So your answer is -1/3. Hope i helped. Have a nice day.