Answer: (-∞, -6].
Step-by-step explanation: since the quadratic function is in vertex form, f(x) = a - (x - h)² + k, where (h, k) represents the vertex of the parabola, we can see that the vertex of this function is (-6, 9).
Since the coefficient of the squared term is negative (-1), the parabola opens downwards, indicating that the function is decreasing on both sides of the vertex.
To make the function one-to-one, we need to restrict the domain to only include the interval where the function is either increasing or decreasing. In this case, we will restrict the domain to the interval where the function is decreasing, which is to the left of the vertex.
Therefore, the restricted domain in interval form is (-∞, -6].