Ardyman, this is not difficult, it's basic geometry.

Tell us if it's a homework or ...

 

 

 

Draw a Circle whose center is at the upper right and whose radius is defined by point A, and another whose center is at the lower left and whose radius is defined by point B.  Select the Polyline and drag the grips at points A and B to the nearer intersection of those two Circles.  Erase the Circles.

 

If you want the resulting Polyline to be closed (in entity-data terms, not just visually), use Pedit and its Edit Vertex option to Break off one of the end segments, move the other end only to that intersection of Circles, and then have the Polyline closed in the Properties box, which will put back the segment you Broke off but adjusted appropriately.

 

It could probably also be done with an AutoLISP Routine that would require only selecting the Polyline, if the same kind of situation arises often enough.

its not so easy for me to do so..connecting those two ends without changing its dimension but all i have to do is to rotate the two lines using their axis. rotating used reference, repeating the command until they met. tnx..