how do you write the demand function when there has been a change in demand? the question asks:

A store is selling t-shirts at $40 per shirt and at this price customers have been buying 60 shirts per month. The owner estimates that for every $1 price increase, 3 fewer shirts will be sold. If the relationship between price per shirt and number sold is linear ...

a) find the demand function expressing price, p, in terms of shirts sold, x

b) find the total revenue R(x) in terms of shirts sold