I need help for geometry

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

What is x3+3x2−16x−48 divided by x−1?

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.