What are residuals and how do i test them against a line of best fit?

The function c(n) below relates the number of bushels of apples picked at a pick-your-own orchard to the final cost for the apples. it takes as input the number of bushels of apples picked after paying an entry fee to an orchard and returns as output and returns as output the cost of the apples (in dollars)c(n) = 15n + 30which equation below represents the inverse function n(c), which takes the cost of the apples as input and returns the number of bushels picked as output