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When the market price is $4, producers will not be willing to furnish any units of the product,
but for each $1 increase in price, producers will be willing to supply an additional 12 units.
Assuming a linear relationship, find the (inverse) supply equation
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What the problem says, is the word description of this linear function
y = 12*(x-4), x >= 4,
where x is the market price per each single unit produced and sold, and y is the number of units.
This function is what the problem requests from you.
Solved and thoroughly expressed.