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

1. Define and explain Third - Degree Price Discrimination;
2. Demonstrate that in order to achieve optimal third - degree price discrimination, marginal revenue for each consumer group must be equal to marginal cost;
3. By using the condition in subpart 2, demonstrate 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.