we know that
If two triangles are similar, then the ratio of its corresponding sides is proportional and its corresponding angles are congruent
In this problem
triangle XRS and triangle TWS are similar by AA similarity theorem
that means
XR/TW=RS/WS=XS/TS
so
RS/WS=XS/TS
multiply in cross
RS*TS=XS*WS
rewrite
RS*ST=WS*SX -----> is proved
