= 2x³ - y + 6x³ - x² + 1
= 8x³ - x² + 1 - y

Thank you for the answer. But why does y move to the back of the equation?

It's not mandatory to move y to the back.

You will also be marked correct if your answer was - y + 8x³ - x² + 1 or 8x³ - y - x² + 1

Moving y to the back is just for grouping all the terms in x to one side to make it more 'neat'

2x³ - y + (2x + 1)(3x² - 2x + 1)

= 2x³ - y + 2x(3x²) + 2x(-2x) + 2x(1) + 1(3x²) + 1(-2x) + 1(1)

= 2x³ - y + 6x³ - 4x² + 2x + 3x² - 2x + 1

= 2x³ + 6x³ - y - 4x² + 3x² + 2x - 2x + 1

= 8x³ - y - x² + 1

(Which also = 8x³ - x² + 1 - y or -y + 8x³ - x² + 1)

Thank you J!

Welcome. Grouping the x terms and y terms is a good practice as it makes the expression clearer/neater. It'll be good to adopt it in your future working