15. (x-1)²/2 + (x-1)/2 - 3 = 0

Let y = (x-1)/2. Then 2y² + 2y - 6 = 0, or y² + y - 3 = 0.

y = (-1 ± sqrt(1 - 4(1)(-3)))/2 = (-1 ± sqrt(13))/2

(x-1)/2 = (-1 ± sqrt(13))/2

x-1 = -1 ± sqrt(13)

x = ±sqrt(13)