Find all numbers c [4,12] for which the derivative is equal to the slope of the secant line connecting the function endpoints. If there is more than one such number c, use a comma to separate your answers. Round to the nearest thousandth. If there is no such number c, type "does not exist" in the entry box.

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?