5. (5 points) For any two numbers $x, y$, the operation “*” is defined as: $x^{*} y=\frac{4 \times \mathrm{x} \times \mathrm{y}}{\mathrm{m} \times \mathrm{x}+3 \times \mathrm{y}}$ (where $m$ is a fixed number). If $1 * 2=1$, then $m=$ $\qquad$ , $3 * 12=$ $\qquad$ .
【Solution】Solution: (1) Since: $x^{*} y=\frac{4 \times x \times y}{m \times x+3 \times y}$ (where $m$ is a certain number)
and $1 * 2=1$
Therefore:
$$
\begin{aligned}
\frac{4 \times 1 \times 2}{m \times 1+3 \times 2} & =1 \\
8 & =m+6 \\
m+6 & =8 \\
m+6-6 & =8 \\
m & =2
\end{aligned}
$$
$$
\begin{array}{l}
\text { (2) } 3 * 12 \\
=\frac{4 \times 3 \times 12}{2 \times 3+3 \times 12} \\
=\frac{144}{42} \\
=\frac{24}{7}
\end{array}
$$

Therefore, the answers are: $2, \frac{24}{7}$.