Can you please explain the following questions for me? I am confused

1. Define and explain Third - Degree Price Discrimination;
2. Show that in order to achieveoptimal third - degree price discrimination, marginal revenue for each consumer group must be equal to marginal cost;
3. Show how the firm carrying out the third - degree price discrimination, should change its prices and total output if the demand curve for one consumer group shifts outward, causing marginal revenue for that group to increase.