PR2=PQ2+RQ2−2(PQ)(RQ)cosQ
→cosQ=PQ2+RQ2−PR2∕2(PQ)(RQ)
→cosQ=82+52−72∕2×8×5
→cosQ=64+25−49∕80
→cosQ=40∕80=0.5
→Q=cos−1(0.5)=600
∴x=600