This is a Paper F5 question – you have posted in the F9 forum!
The BPP answer is correct.
For every $5 change in price, the demand changes by 10,000 units. That is exactly the same as saying that for every $1 change in price, the demand changes by 10,000/5 = 2,000 units.
However you write it, ‘b’ in the equation is still equal to 0.0005