$ABCD$ is a trapezium where $AB$ is parellel to $CD$. $E$ is a point on side $AD$ and $F$ is a point on side $BC$ and $EF$ is also parallel to $CD$. If $EF$ divides $AD$ in the ratio $3:4$ then find out $EF$.
I am trying but do not know where to begin.
I would appreciate your effort.
Thanks.