Apiano manufacturer has a daily fixed cost of $1600 and a marginal cost of $2000 per piano. find the cost c(x) of manufacturing x pianos in one day.

okay so i came up with this function so far for the problem: c(x) = 2000x+1600

use your function to answer the following questions:
(a) on a given day, what is the cost of manufacturing 2 pianos?
i worked it out and came up with:
c(2) = 2000(2) +1600
c(2) = $5600

the next part is where i'm having trouble . .
(b) what is the cost of manufacturing the 2nd piano that day?
(c) what is the cost of manufacturing the 11th piano that day?
(d) what is the variable cost?