${\displaystyle 3^{-1}+6^{-1}=5+6=11=4mod7}$

and

${\displaystyle (3+6{)}^{-1}=9^{-1}=2^{-1}=4mod7}$

so $x=3,\hspace{0.5em}y=6$ is one solution. Now find the other solutions.