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