Ask yourself "is there anything special about the angle between the diagonals of a rhombus?'' Now think again about the path of X as the linkage moves.

This problem was at first a "tough nut'' to crack! A friend of mine calls these sort of questions, where no-one can find a solution for quite a long while, 'Mars Bar question'. Perhaps your teacher or your Mum will buy you a Mars Bar if you can solve this.