The cost function is linear, and as such has a slope and a y-intercept.
Two points on this line are (5, $1750) and (10, $3000). As the number of heaters produced increases from 5 to 10, the production costs increase by $1250. Thus, the slope of this line is m = $1250/5, or m = $250/unit.
Inserting known data into the slope-intercept form of the equation of a straight line, we get:
$3000 = ($250/unit)(10) + b. Then $500 = b, and the cost function is:
y = ($250/unit)(x) + $500.
