You can obtain an explanation for any step by clicking on it. Please note that you cannot click on the very first expression, which is the original problem. In particular, for line 4, we use the formula for difference of cubes:

A^3 - B^3 = (A - B)(A^2 + AB + B^2)

Instead of 'A' put 'x' and instead of 'B' put '3', you will get:

x^3 - 3^2 = (x - 3)(x^2 + 3x + 3^2)