b
\(\begin{array}{l}
\overrightarrow S = \left( {5\hat i + 4\hat j} \right)2 + \frac{1}{2}\left( {4\hat j + 4\hat j} \right)4\\
\,\,\,\, = \,10\hat i + 8\hat j + 8\hat i + 8\hat j\\
\overrightarrow {{r_2}} - \overrightarrow {{r_1}} = 18\hat i + 16\hat j\\
\overrightarrow {{r_2}} = 20\hat i + 20\hat j\\
\left| {\overrightarrow {{r_f}} } \right| = 20\sqrt 2
\end{array}\)