Answer
$\dfrac{2m}{m-4}$
Work Step by Step
The given expression, $
\dfrac{m}{4-m}+\dfrac{3m}{m-4}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{m}{4-m}+\dfrac{3m}{-(-m+4)}
\\\\=
\dfrac{m}{4-m}-\dfrac{3m}{4-m}
\\\\=
\dfrac{m-3m}{4-m}
\\\\=
\dfrac{-2m}{4-m}
\\\\=
\dfrac{-2m}{-(-4+m)}
\\\\=
\dfrac{2m}{m-4}
.\end{array}
