Point \(X\) is the midpoint of the diagonal \(AC\) of a convex quadrilateral \(ABCD\). It is known that \(CD \| BX\). Calculate the length of \(AD\) if \(BX=3\), \(BC=7\), and \(CD=6\).

Punkts \(X\) ir izliekta četrstūra \(ABCD\) diagonāles \(AC\) viduspunkts. Zināms, ka \(CD \| BX\). Aprēķināt \(AD\) garumu, ja \(BX=3, BC=7\) un \(CD=6\).