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