How create elastic string for launching objects?

So I would like to create a string that has elastic properties. This string would be attached two to objects (anchors) and the idea is to drag object that is above the string, pull down which will tighten the string and once I release the object the string will launch it. Similar effect as slingshot.

In the left picture the string is in normal position, and player drags pink circle down, which makes string to expand (right picture). Release left mouse click (which was used for dragging) will make string to retract back to normal position also providing launch force to pink object.

How can I achieve this effect. Also the idea is for this to be 3D game with top down view.