At what point...

[PrudensOptimus]

At what point is the tangent to f(x)=x^2 + 4x - 1 horizontal?

How do you do that? I think it's asking me at what point is x = 0, or undefined, right??

[Hurkyl]

Hint:

What is the unique feature about the equation of a horizontal line?
How does that relate to tangent lines?

[mathman]

Horizontal tangent is where derivative=0, that is x=-2.