Answer
$\frac{3m(2-\sqrt {m+n})}{4-m-n}$
Work Step by Step
Multiply $\frac{2-\sqrt {m+n}}{2-\sqrt {m+n}}$ to the expression, we have:
$\frac{3m}{2+\sqrt {m+n}}\cdot \frac{2-\sqrt {m+n}}{2-\sqrt {m+n}}=\frac{3m(2-\sqrt {m+n})}{4-m-n}$
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