Answer
$x=-2$
Work Step by Step
The derivative of a function is the slope of the tangent line of that curve. In the picture we derivate the function and get the slope (in function of x). Of course the tangent line touches a point of the curve, let's call it $(x_{o},y_{o})$.
Then we write the tangent line equation, making $x=y=0$, assuring the tangent line is passing through the origin. Writing $y_{o}$ in function of $x_{o}$, we can solve the equation for $x_{o}$, finding the only solution, $-2$.
