Let r be the radius of the ferris wheel. Then you should be able to find an expression for the height of the seat being observed as a function of time. It will also give you the distance from the center line, and from that and 20m you can get an expression for the horizontal distance to the seat being observed. Then you can finally get theta as a function of time, so you take the derivative and set it to zero.
If I understand the problem correctly, the angle theta is between the line of sight and the horizontal plane, i.e. how far it is above the horizon, ignoring the side-to-side motion, so the diagram is a little deceptive.