Question 345457
{{{(s+t)(s^2-st+t^2)=s(s^2-st+t^2)+t(s^2-st+t^2)}}}
{{{(s+t)(s^2-st+t^2)=(s^3-s^2t+st^2)+(s^2t-st^2+t^3)}}}
Take it from there, combine like terms, and come up with the final answer.