The magnitude of the resultant of four forces $\overrightarrow{P}=\overrightarrow{i} - \overrightarrow{j} - \overrightarrow{k}; \overrightarrow{Q}=\overrightarrow{i}-2\overrightarrow{j}+2\overrightarrow{k}; \overrightarrow{R}= \overrightarrow{i}- \overrightarrow{j}+3\overrightarrow{k}; \overrightarrow{S}= \overrightarrow{i}+b\overrightarrow{j}$ acting on a particle is $6$ if $6 $ is equal to
