Height of the object, \(H_{0}=3 cm\),
distance of the object \(=u=12 cm\)
focal length of the lens \(=8.0 cm\)
using lens equation,
$$
\begin{aligned}
&\frac{1}{f}=\frac{1}{u}+\frac{1}{v}, \frac{1}{8}=\frac{1}{12}+\frac{1}{v} \\
&\frac{1}{v}=\frac{1}{8}-\frac{1}{12} ; \frac{1}{v}=\frac{1}{24}, v=24 cm
\end{aligned}
$$
Recall that;
$$
\frac{H_{i}}{H_{0}}=\frac{v}{u}=m
$$
Where \(H_{ i }\) and \(H_{ o }\) are height of the image and object respectively.
$$
\frac{H_{1}}{3}=\frac{24}{12}, H_{1}=6 cm
$$