The problem states:

In planning for a new item, a manufacturer assumes that the number of items produced X and the cost in dollars C of producing these items are related by a linear equation. Projections are that 100 items will cost $10,000 to produce and that 300 items will cost $22,000 to produce. Find the equation that relates C and X.

This is what i did:
THe two points relating C and X: (100,10000) and (300,22000)

m= (22000-10000)/(300-100)=
(12000)/(300)=60

Using one of the point values for x and C and m=60 i can then solve for "B"

10000=60(100)+b
b=10000-6000=4000

Equation would be: Cost = 60x+4000

First of all, the slope is 60 but you typed (12000)/(300) and it should be (12000)/(200). The rest is right.