Between Steps 1 and 2, the student added 6x to the left hand side, but subtracted 6x from the right hand side.
x^6 + 10ax^3 + 2ax^3 + 20a^2
(x^3 + 10a)(x^3 + 2a)
answer: (x^3 + 10a)(x^3 + 2a)
Hope it helps :)
