Answer:
(x +5)
Explanation:
The problem statement is telling you that one factor of (x⁴ +5x³ -3x -15) is (x³ -3). It is asking for the other factor. Clearly, you can find the other factor by dividing the polynomial by the given factor.
That is ...
(x⁴ +5x³ -3x -15) / (x³ -3) = (x +5)
so ...
(x⁴ +5x³ -3x -15) / (x +5) = (x³ -3)
The divisor of interest is (x +5).