8.

a. The fi rst step is to set QD = QS. Doing so gives us 90 - 2P = P. Solving for price, we fi nd that 90 = 3P, or P = 30. Once we know that P = 30, we can plug this value back into either of the original equations, QD = 90 - 2P or QS = P. Beginning with QD, we get 90 - (30) = 90 - 60 = 30, or we can plug it into QS = P, so QS = 30. Since we get a quantity of 30 for both QD and QS, we know that the price of $30 is correct.