x(x + 1)(x - 1)
is the same as
x[(x + 1)(x - 1)], so lets do what is inside the brackets first.
Using FOIL method (FOIL is First Outer Inner Last, ask me if you need clarification)
x[x^2 - x + x - 1]
- x + x cancels, now distribute the x
x^3 - x <---ANSWER