In the second line you have:
( x + 64 ) ( x² - 64 x + 64² )
and it should be:
( x + 4 ) ( x² - 4 x + 4² )
because: ( A³ + B³ )= ( A + B ) ( A² - AB + B² )
You have: B³ = 64 so B = ∛64 = 4
After that:
x + 4 = 0
x = - 4
We can justify it:
( - 4 )³ + 61 = 0
- 64 + 64 = 0 ( correct )
