\({\overrightarrow r _{cm}} = \frac{{2m\,{{\overrightarrow r }_1} + m{{\overrightarrow r }_2} + m{{\overrightarrow r }_3}}}{{4\,m}}\)
\({\overrightarrow r _{cm}} = \frac{{2m\left( {L\hat i + L\hat j} \right) + m\left( {2L\hat i + \frac{L}{2}\hat j} \right) + m\left( {\frac{{5L}}{2}\hat i} \right)}}{{4\,m}}\)
\( = \frac{{\frac{{13}}{2}L\hat i + \frac{5}{2}L\hat j}}{4}\)
\( = \frac{{13}}{8}L\hat i + \frac{5}{8}L\hat j\)