Ineed finding the angel relationship

Autility company burns coal to generate electricity. the cost c(p), (in dollars) of removing p percent of the some stack pollutants is given by c(p)= 85000p/100-p a) find the cost to remove 30% of the pollutant b) find where the function is undefined

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

What is the slope of the line shown below?