I would write for the angle between AB and the horizontal (given as 56 degrees in the diagram), and similarly for the angles that BD and DE make with the horizontal. Also, write x for the length of DE. Then the horizontal and vertical distances from A to E remain constant as the system moves. Therefore
Differentiate these equations with respect to time:
Now plug in the values You can eliminate between the two equations, and you're left with an equation for (which is the angular velocity of the plate, in a clockwise direction).