Hi 
(500,15000)    |Using the point-slope formula, 
(100,5000)  m = 10,000/400 = 25
Using the standard slope-intercept form for an equation of a line y = mx + b
  where 25 is the slope and b the y-intercept.  
 C(x) = 25x + b
 5000 = 25*100 + b   |Using ordered pair (100,5000) to solve for b
 2500 = b
 C(x) = 25*x + $2500    
 C(400) = 25*400 + $2500 = $12,500
$18,000 = 25x + $2500
 15500/25 = x = 620 items would be predicted to be produced with $18,000