Let side of $\mathrm{AB}$ and $\mathrm{BC}$ is a
$$
\begin{aligned}
& \mathrm{a}^2+\mathrm{a}^2=(10)^2 \mathrm{~cm} \\
& 2 \mathrm{a}^2=(10)^2 \\
& \mathrm{a}=\frac{10}{\sqrt{2}}
\end{aligned}
$$
The magnitude of dipole moment
$$
\begin{aligned}
& P=2 q a \cos \theta=2 q \times \frac{10}{\sqrt{2}} \times \cos 45 \\
& =2 \varepsilon \times \frac{10}{\sqrt{2}} \times \frac{1}{\sqrt{2}}=10 \mathrm{~cm}
\end{aligned}
$$