Here, 3m is a scalar that we use to multiply each element of the matrix.
We have
[tex] 3m \left[\begin{array}{cc}-5m&2m\\6m&6n\end{array}\right] [/tex].
We get
[tex]\left[\begin{array}{cc}(3m)(-5m)&(3m)(2m)\\(3m)(6m)&(3m)(6n)\end{array}\right] [/tex]
which simplifies to
[tex]\left[\begin{array}{cc}-15m^2&6m^2\\18m^2&18mn\end{array}\right] [/tex]
