Corry,
the common factor for both are (t-u), we can take it out:
(t-u)(s^2-9t^2)
And because a^2-b^2=(a+b)(a-b), in this case,
s^2-9t^2
=s^2-(3t)^2 [note: s=a, 3t=b in the above formula]
=(s+3t)(s-3t)
So the complete factor for this one is (t-u)(s+3t)(s-3t)
Hope it helps.
Heng