For two triangles PQR and XYZ, PQQR=YZZX and QRPR=ZXYX .
PQQR=YZZX
⇒PQYZ=QRZX ----------- I
QRPR=ZXYX
⇒QRZX=PRYX -----------II
From I and II,
PQYZ=QRZX=PRYX
∴△PQR∼△YZX by SSS axiom.
If PQQR=YZZX and QRPR=ZXYX , then