Can someone help me on this

Evaluate the expression for the given value of the variable 2×(c2-5) for c=4

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

Lin is writing an equation to model the proportional relationship between y, the total cost in dollars of downloading videos from a website, and x, the number of videos downloaded. she knows that the total cost to download 3 videos was $12. her work to find the equation is shown below. joylin’s work step 1 k= 3/12= 0.25 step 2 y= 0.25x where did joylin make her first error?