Find all points in the open interval (a, b) where the tangent line to y = f(x)

Question

is parallel to the secant line....?

Find all points in the open interval (a, b) where the tangent line to y = f(x) is
parallel to the secant line joining (a, f(a)) and (b, f(b)) when f(x) = x^3 − 3x + 1
with domain [−2, 2].