Dividend flow has Multi-stage growing Pattern

Twitter Inc. is projected to grow at 50% per year for 3 years. By then, other companies will have similar service or technology, the competition will drive down its profit margin and the sustainable growth rate will fall to 4%. The most recent annual dividend was $1 per share. The required return of Twitter is 12%.

a. What are the expected values of D1, D2, D3 and D4?

b. What is the expected stock price 3 years from now?

c. What is the intrinsic value of this stock today?

d. What is the expected price of this stock one year later?

can i have more calculation details or reasons? thanks!