(a) For a perfectly competitive market, a rotation in the demand curve from D₁ to D₂ does not change the equilibrium quantity Q*_c and price Q*_c.
(b) For a firm with market power, a rotation in the demand curve from D₁ to D₂ rotates the marginal revenue curve from MR₁ to MR₂. Prior to the rotation, the profit-maximizing quantity and price (Q*_m1, P*_m1) occurred where MR₁ = MC. After the rotation, the firm is profit-maximizing at a higher quantity and lower price (Q*_m2, P*_m2) where MR₂ = MC.