Can someone please answer this real quick

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?

Find the quotient with the restriction.

Matt had 5 library books. he checked out 1 additional book every week without retuening any books. whats the matching equation