Find points at which the tangent to the curve `y=x^3-3x^2-9x+7`is parallel to the x-axis.

Explanation

To find the points at which the tangent to the curve \( y = x^3 - 3x^2 - 9x + 7 \) is parallel to the x-axis, we follow these steps:

### Step 1: Understand the condition for the tangent to be parallel to the x-axis.
A tangent line is parallel to the x-axis when its slope is zero. Therefore, we need to find where the derivative of the function is equal to zero.

### Step 2: Differentiate the function.
We differentiate the function \( y = x^3 - 3x^2 - 9x + 7 \):
\[
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