Algrebra help please

So I need to factor these and I'm having trouble help please
81x^4-16y^4
And
s^4-625t^4

81x^4 - 16y^4 = (9x^2 + 4y^2) (9x^2 - 4y^2). That further simplifies to:

(9x^2 + 4y^2) (3x + 2y) (3x - 2y)

The second problem is similar:

s^4 - 625t^4 = (s^2 + 25t^2) (s^2 - 25t^2). That further simplifies to:

(s^2 + 25t^2) (s + 5t) (s - 5t)