Quantitative analysis
posted by Rebekah .
If we produce 350 units of output, the total cost is $11,500. It costs us $12,900 to produce 420 units. If we assume the relationship is linear, determine the cost function.

treat your data as two ordered pairs
( 350 , 11500) and (420 , 12900)
find the equation like you would given any two ordered pairs,
i.e.
find the slope etc 
cost = slope (number of units) + constant
y = m x + b
11500 = 350 m + b
12900 = 420 m + b
solve for m and b by elimination or substitution