What should be added to 2a³ + a² - 3a - 1 to get 7 + 2a - a² + 4a³?

2a³ - 2a² + 5a + 8

4a³ - 2a² + 5a + 8

5a³ - 3a² + 5a + 8

None of these

Let's calculate what needs to be added:

(7 + 2a - a² + 4a³) - (2a³ + a² - 3a - 1)

= 7 + 2a - a² + 4a³ - 2a³ - a² + 3a + 1

= 2a³ - 2a² + 5a + 8