r^3S^3 IS THE SAME AS (rs)^3. Similarly, t^6 is the same as (t^2)^3. Therefore, the given binomial is the difference of two cubes. Using the "difference of two cubes" formula,
r^3s^3-t^6 becomes (rs)^3 - (t^2)^3 = (rs - t^2) - [(rs)^2 + rst^2 + (t^2)^2]