If f'(x)=tan^-1(x^3-x) at how many points is the tangent line to the graph of y=f(x) parallel to the line y=2x

The slope of the line is 2, and any parallel line will have the same slope. Since gives the tangent line to at , any tangent line will be parallel whenever



But this never happens, since

So there will never be a tangent line with slope 2, and thus no tangent line parallel to .

They are somewhat similar