Use the remainder theorem and the factor theorem to determine whether (y − 3) is a factor of (y4 + 2y2 − 4).

Since the factor is (y-3) then we can say y - 3 = 0, therefore y  = 3.  Thus the root or zero is at 3, using that factor.

Now, is it a factor of y⁴ +2y² - 4 ?

using the Remainder Theorem, that is true only if f(3) = 0, let's check then,

f(y) = y⁴ +2y² - 4 

f(3) = (3)⁴ +2(3)² - 4 

f(3)  = 81 + 54 - 4

f(3) = 131

since f(3) ≠ 0, then the factor with the root of "3" is not a factor of y⁴ +2y² - 4.

The length is 23 and y because you add them together