I'm assuming that the question is "Given the function "f(x) = x^3 - 15x", what are the points of inflection?
Points of inflection are the zeros of the 2nd derivative of a function. For this function, the derivatives are:
f(x) = x^3 - 15x <- original function
f'(x) = 3x^2 - 15 <- 1st derivative
f''(x) = 6x <- 2nd derivative
Since the 2nd derivative has only one zero solution of "x = 0", the original function has x=0 as the inflection point.
