\[
\boxed{\text{No tangent line can have slope 6.}}
\]
We need to find the points on the parabola \( y = x^2 \) where the tangent lines pass through a given point. Let's assume the point is \( (a, b) \).
The equation of the tangent line to the parabola \( y = x^2 \) at a point \( (x_1, y_1) \) is:
\[
y - y_1 = m(x - x_1)
\]
where \( m \) is the slope of the tangent line. For the parabola \( y = x^2 \), the slope at \( x_1 \) is \( 2x_1 \).
So, the equation becomes:
\[
y - x_1^2 = 2x_1(x - x_1)
\]
We need to find the points \( (x_1, x_1^2) \) such that the tangent line passes through \( (a, b) \).
\[
b - x_1^2 = 2x_1(a - x_1)
\]
Rearrange and solve for \( x_1 \):
\[
b - x_1^2 = 2ax_1 - 2x_1^2
\]
\[
b = 2ax_1 - x_1^2
\]
\[
x_1^2 - 2ax_1 + b = 0
\]
This is a quadratic equation in \( x_1 \). Solving for \( x_1 \):
\[
x_1 = \frac{2a \pm \sqrt{(2a)^2 - 4 \cdot 1 \cdot b}}{2 \cdot 1}
\]
\[
x_1 = a \pm \sqrt{a^2 - b}
\]
\[
\boxed{(x, y) = (a + \sqrt{a^2 - b}, (a + \sqrt{a^2 - b})^2)}
\]
\[
\boxed{(x, y) = (a - \sqrt{a^2 - b}, (a - \sqrt{a^2 - b})^2)}
\]
The function \( f(x) = |x^2 - 4| \) is differentiable wherever the expression inside the absolute value is not zero.
Set the expression inside the absolute value to zero:
\[
x^2 - 4 = 0
\]
\[
x^2 = 4
\]
\[
x = \pm 2
\]
The function \( f(x) \) is differentiable on the intervals where \( x^2 - 4 \neq 0 \):
\[
(-\infty, -2) \cup (-2, 2) \cup (2, \infty)
\]
\[
\boxed{(-\infty, -2) \cup (-2, 2) \cup (2, \infty)}
\]