you have to add 1 to the -1 and you will get 0.What you have to do next is also add 1 to 15 because you want a variable on one side and a number on the other.So now you have 8k=16. So you divide 8 to 8k and you get nothing divide 8 and 16 and your answer for k =2 hope you get it right.trust mw it is very easy when you get use to it