I need help understanding how to do the second part, thanks! :)

The management of Primo's Prime Pizzeria is considering a special promotion for the last two weeks of may, which is normally a relatively low demand period. The special promotion would involve selling two medium pizzas for the price of one plus lc. The medium pizza normally sells for S9.99 and has variable costs of S3.00. Expected sales volume without the special promotion is 400 medium pizzas per week. (a) Calculate the total contribution margin generated by the normal sales volume of medium pizzas in a weelk REQUIRED: (b) Calculate the total number of medium pizzas that would have to be sold during the l sale to generate the same amount of contribution margin that results from the normal sales volume