<p>As PWN said, you need to get m all by itself.</p>
<p>5<em>m^5</em>n^-3 = 20<em>m^3</em>n</p>
<p>m^5<em>n^-3 = 4 *m^3</em>n (Divide both sides by 5)</p>
<p>m^2<em>n^-3 = 4</em> n (Divide both sides by m^3)</p>
<p>m^2 = 4*n^4 (Multiply both sides by n^3)</p>
<p>m = 2n^2 (take square roots of both sides but take only positive root, since m is positive)</p>