Clicking on a particular solution step is a good tool for explanations.  As shown in the attached image, x^3 and 125 are both cubed as x^3 and 5^3, hence "x^3+5^3" can be factored into (x+5)(x^2 - 5x + 25). 

It is also worth noting that 'X' is a different variable than 'x', so if you are ever entering in an expression with both 'x' and 'X' entries then they will be interpreted as different variables.