Given that the quantity demanded for a certain brand of portable CD players is 200 units when the unit price is set at $90.
If we set quantity=y and price=x then in point form, we can write above information as (90,200)
Similarly "The quantity demanded for 1,200 units when the unit price is set at $40. " can be written as (40,1200)
Now we can plug both points into two point formula to find the required equation:
plug those points
y-1200=-20(x-40)
y-1200=-20x+800
y=-20x+2000
Hence demand equation is y=-20x+2000