The sum of two forces $\overrightarrow { \mathrm { P } }$ and $\overrightarrow { \mathrm { Q } }$ is $\overrightarrow { \mathrm { R } }$ such that $| \overrightarrow { \mathrm { R } } | = | \overrightarrow { \mathrm { P } } |$. Find the angle between resultant of $2 \overrightarrow { \mathrm { P } }$ and $\overrightarrow { \mathrm { Q } }$ and $\overrightarrow { \mathrm { Q } }$.