What must be added to 7x³ – 3x² +6x + 4 to make the sum x³ – 5x²- x + 1 ?

Let us take unknown value as A.
(7x³ - 3x² + 6x + 4) + A =(x³ - 5x² - x + 1)
So, A = ​(x³ - 5x² - x + 1) - (​7x³ - 3x² + 6x + 4)
= x³ - 5x² - x + 1 - 7x³ + 3x² - 6x - 4
Group like terms together
= x³  - 7x³​ - 5x² + 3x² ​- x ​- 6x + 1 - 4
= - 6x³ - 2x² - 7x - 3
A = (​- 6x³ - 2x²​ - 7x - 3)

@ Tarun Raghavend...- Good attempt