Explanation
$$\Rightarrow $$ If $$x\leq 0$$, then $$x^{12}-x^9+x^4-x+1>0$$
$$\Rightarrow $$ If $$0<x\leq 1$$, then $$x^{12}+x^4(1-x^5)+(1-x)>0$$
$$\Rightarrow $$ If $$x>1$$, then $$x^9(x^3-1)+x(x^3-1)+1>0$$
So the expression $$x^{12}-x^9+x^4-x+1>0$$ is valid for $$-\infty<x<\infty$$
We have to find the velocity along shoreline or parallel to river.
We can see in figure that the velocity V makes an angle $$(45+15) =60^0$$ with the river.
So,
$$V\,cos\theta = V\, cos60^0$$
$$= 18\times \dfrac12$$
$$=9\, km/hr$$
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