I have a solution to a problem, but I am struggling to understand where part of the solution comes from. How is 'Q' derived?

Example:

Weekend

P= 15-0.001Q

Total Revenue = Price * Quantity

Total Revenue = 15Q - 0.001Q2

Marginal Revenue = 15 - 0.002Q

Marginal Cost = 2

Marginal Revenue = Marginal Cost

15 - 0.002Q = 2

Q = 6500

P= 15 - 0.001(6500)

P = $8.50