An e-commerce firm selling mobile phone accessories decides to invest in advertising to boost the sales. The firm is expected to sell $25 million worth of the product. It is estimated that a 1 % increase in the advertising budget would increase quantity sold by 6 %. Moreover, it is also estimated that a 1 % increase in the product's price would reduce quantity sold by 0.3 %.

a. Derive the Dorfman-Steiner condition.

b. Calculate the optimal advertising budget for this firm.