Let  $f\left(x\right)=a(x-2)(x-b),$  where  $a,b\in R$ and  $a\ne 0.$ Also  $f\left(f\left(x\right)\right)=a^3(x^2-(2+b)x+2b-\frac{2}{a})(x^2-(2+b)x+2b-\frac{b}{a}), a\ne 0,$  has exactly one real root as 5. The slope of the straight line passing through  $O(0,0)$ and tangent to  $y=f\left(x\right),$  can be