What does rockefeller promise vanderbilt that he may not be able to keep?

Rocketfeller wasn’t born rich but he is poor as he was born but as he became successful and was able to attain riches when he grew. The promise that he had said to Vanderbilt that he wasn’t able to keep was that he promise he will provide him sixty train cars each day but he wasn’t able to provide it.

8 - (2a + 7) = a + 40     distribute/multiply - into (2a + 7)

8 - 2a - 7 = a + 40

1 - 2a = a + 40       add 2a on both sides

1 = 3a + 40         subtract 40 on both sides

-39 = 3a       divide 3 on both sides

-13 = a

[proof]

8 - (2a + 7) = a + 40

8 - (2(-13) + 7) = -13 + 40

8 - (-26 + 7) = 27

8 - (-19) = 27

8 + 19 = 27

27 = 27