I am having trouble to formulate the demand function for this problem.

The demand function for a certain product is determined by the fact that the product of the price and the quantity demanded equals 8000. The product currently sells for $3.50 per unit. Suppose manufacturing costs are increasing over time at a rate of 15% and the company plans to increase the pricepat this rate as well. Find the rate of change of demand over time.

I know this is a related rate question. I just, for some reason, can't figure it out how to get the demand function.