factoring completely
250x3-128y3
x2+100
$\displaystyle 250x^3 - 128y^3 = 2(125x^3 - 64y^3) = 2[(5x)^3 - (4y)^3]$
now use the difference of two cubes formula: $\displaystyle a^3 - b^3 = (a - b)(a^2 + ab + b^2)$
here of course, your $\displaystyle a$ is $\displaystyle 5x$ and your $\displaystyle b$ is $\displaystyle 4y$