A bookstore has a linear demand function for stationary. when the price of the note card is $4, customers are willing to buy 84 packages. when the price is $7 customers would buy 72 packages.

a) find an equation q=f(p) for the demand. use descriptive variables, i.e. p and q.

b) assume the supply function is given by q=16p. find the equilibrium price and quantity.