Answer
$m = \frac{2P-6}{n^{2}}$
Work Step by Step
$P = \frac{1}{2}mn^{2} +3$
$P = \frac{mn^{2}}{2} +3$
$P-3 = \frac{mn^{2}}{2}$
$2(P-3) =mn^{2}$
$2P-6 = mn^{2}$
$m = \frac{2P-6}{n^{2}}$
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