Answer :
The answer is x^3+15x^2+75x+125
Work: (x+5)^3 , x^3+3x^2*5+3x*5^2+5^3 , 3x*25+125+x^3+15x^2 and you get x^3+15x^2+75x+125
Work: (x+5)^3 , x^3+3x^2*5+3x*5^2+5^3 , 3x*25+125+x^3+15x^2 and you get x^3+15x^2+75x+125