Suppose that 10,000 units of a certain item are sold per day by the entire industry at a price of $150 per item, and that 8000 units can be sold per day by the same industry at a price of $200 per item. Find the demand equation for

in terms of

, the number of units sold, assuming the demand curve to be a straight line.

I know the answer will be a straight line, so P(x)=ax+b

and I know

if you sell 10000 units, price becomes $150 so in price equation above you get 150=a(10000)+b

and

if you sell 8000 units, price becomes $200 so the equation becomes 200=a(8000)+b

I just can't figure out how to solve to get A and B pretty much...