To find the point of inflection, we need to find the second derivative.
Since we are discussing cubic function, the assumption is that a is not zero. Thus for any cubic function of the given form, we have the inflection point at `x=-b/(3a)`
To find the slope at this point, we need to plug in the value of x in the first derivative function.
Thus the slope is `-(b^2)/(3a)+c`