SI. 3 In figure $1, A B=A C$ and $K L=L M$. If $L C=Q-6 \mathrm{~cm}$ and $K B=R \mathrm{~cm}$, find the value of $R$.
Draw $L N / / A B$ on $B M$.
$B N=N M$ intercept theorem
$$
\begin{array}{l}
\angle L N C=\angle A B C=\angle L C N \text { (corr. } \angle \mathrm{s}, A B / / L N, \text { base } \angle \mathrm{s} \text {, isos. } \triangle) \\
L N=L C=Q-6 \mathrm{~cm}=1 \mathrm{~cm} \text { (sides opp. eq. } \angle \mathrm{s}) \\
R \mathrm{~cm}=K B=2 L N=2 \mathrm{~cm} \text { (mid point theorem) }
\end{array}
$$