How to find the surface area of a rectangle with only the volume and the length.

I believe that you'd need to know what either the height or the width is, because otherwise the prism's surface area could vary.

$V_{rectangle}=L\times W\times H \\ S_{rectangle}=2(lw + lh + wh)$

it s only possible if

$H=W$

So you have length=12 and Volume=324:

$324=12\times W \times H \\ W=\sqrt{27}\\ H=\sqrt{27}$

$S_{rectangle}=2(12\sqrt{27}+12\sqrt{27}+\sqrt{27} \times\sqrt{27})=303.4153162899183302679522731768456208397639565486948104400\dots$