need help i do not understand

Question

The total cost of printing small booklets is  (the fixed cost) plus  for each booklet printed. Express the total cost of producing booklets, , as a function of the number of booklets produced, .

Answer 1

For 1 booklet = 3(1)

For 2 booklets = 3(2)

For 3 booklets = 3(3)

...

For c booklets = 3(c)

then additional $600 so:

X(c) = 3c + 600