Given the polynomial 21x 4 + 3y - 6x 2 + 34 what polynomial must be subtracted from it to obtain 29x 2 - 7 ?

The given polynomial is 21x⁴ + 3y - 6x² + 34

We want to subtract a polynomial, f(x) so that
21x⁴ + 3y - 6x² + 34 - f(x) = 29x² - 7

Therefore
-f(x) = 29x² - 7 - (21x⁴ + 3y - 6x² + 34)
       = -21x⁴ + 35x² - 3y - 41

21x⁴ - 35x² + 3y + 41