The cube of a binomial (sum) is equal to the cube of the first term, plus three times the square of the first by the second, plus three times the first by the square of the second, plus the cube of the second term. In other words: $(a+b)^3=a^3+3a^2b+3ab^2+b^3 = (5x^8)^3+3(5x^8)^2(2y^3)+3(5x^8)(2y^3)^2+(2y^3)^3 =$

The power of a product is equal to the product of it's factors raised to the same power