I have the equation -tan(x)=x.

I have no idea how to solve it.

The above comes from the derivative of x*sin(x), which is x*cos(x)+sin(x). If we try to find the extremums of x*sin(x) we have:

x*cos(x)+sin(x)=0

$\displaystyle

x=-\frac {sin(x)}{cos(x)}

$

x=-tan(x)