Question 164277
Marginal cost means additional cost for additional item which is $40/item. But this is exactly the slope. You have another information that (70, 3800) is on the line.
So by the point slope formula 
y-y_1=m(x-x_1)
we have
C-3800= 40(x-70)
C(x)=40(x-70)+3800
C(x)=40x +1000
So the answer ============> (b)

The given two data about the sales in 1982 and 1987 tell us that the point
(1982, 10,500) and (1987, 67,000) are on the line.
The slope then equals (67,000 - 10,500)/(1987 - 1982) =56,500/5=11300.
We have
S(x)=11,300 x+b
since x=0 correspond to 1982 and we now that S=10,500 in 1982 then
10,500 =11,300 (0)+b ========> b=10,500 and therefore
S(x)=11,300x+ 10,500.

More Help?
Visit http://www.watchmath.com