Explanation

Since x = 2 is a zero, p(2) = 8 + 12 − 12 − a = 0 gives a = 8. Dividing x³ + 3x² − 6x − 8 by (x − 2) leaves x² + 5x + 4 = (x + 1)(x + 4), so the other zeros are −1 and −4. Their squares add to 1 + 16 = 17, option (b).