How do i do 1/2 plus -4 plus 8 minus -1/2

Me. i can't figure out this question. give an explanation too. .

Can you me find the missing length. i attached an image.

Kayla made observations about the sellin price of a new brand of coffee that sold in the three different sized bags she recorded those observations in the following table 6 is $2.10 8 is $2.80 and 16 is to $5.60 use the relationship to predict the cost of a 20oz bag of coffee.