Problem: <image>In $\triangle PQR$, $ZQ=3a-11$, $ZP=a+5$, $PY=2 c-1$, $YR=4 c-11$, $m \angle PRZ=4 b-17$, $m \angle ZRQ=3 b-4$, $m \angle QYR=7 b+6$, and $m \angle PXR=2 a+10$.  Find $PR$ if $\overline{QY}$ is a median.
Answer: 18