Question 172490
The following problem was answered yesterday.  I think it relates 
to your problem.
Cheers,
Stan H.
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Suppose that a market research company finds that at a price of p = $20, they would sell x = 42 tiles each month. If they lower the price to p = $10, then more people would purchase the tile, and they can expect to sell x = 52 tiles in a month’s time. Find the equation of the line for the demand equation. Write your answer in the form p = mx + b. (Hint: Write an equation using two points in the form (x,p)). 
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You have two points: (42,20) and (52,10)
slope = (10-20)/(52-42) = -1
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intercept: p = mx + b; solve for "b" as follows:
20 = -1(42) + b
b = 20 + 42
b = 62
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EQUATION:
p = -x + 104
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Cheers,
Stan H.