Explanation
$$F(x)$$ is clearly continuous and differentiable at $$x = 0$$ zero with $$f`(0) = 0$$.
$$\displaystyle f\, (0)\, =\, \lim_{h \rightarrow 0} \frac{3h^2\, \sin \frac{1}{h}\, -\, h\, \cos \frac{1}{h}}{h}$$ $$\displaystyle =\, 3h\sin\frac{1}{h}\, -\, \cos \frac{1}{h}$$
This limit doesn't exist, hence $$f(x)$$ is non-differentiable at $$x = 0$$.
Also $$\displaystyle \lim_{x \rightarrow 0}\, f`(x)\, =\, 0$$.
Thus $$f`(x)$$ is continuous at $$x = 0$$.
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