Answer:
The remaining solutions are -7 and 3.
Step-by-step explanation:
The synthetic division is shown in the attachment. After dividing the first two found factors from the polynomial, the remaining quadratic is ...
x^2 +4x -21 = 0
Its rational roots will be factors of -21, and the sum of their opposites will be 4. Knowing that 21 = 3·7, we can guess that the remaining roots are -7 and 3. The synthetic division confirms that. (A graph also confirms it.)
