9514 1404 393
Answer:
 C = 9x +2280
Explanation:
The 2-point form of the equation for a line can be a place to start.
 y = (y2 -y1)/(x2 -x1)(x -x1) +y1
 C = (4800 -3000)/(280 -80)(x -80) +3000 . . . . . use given values
 C = 1800/200(x -80) +3000 . . . . simplify some
 C = 9x -720 +3000 . . . . eliminate parentheses
 C = 9x +2280