I won't do all of your work for you, but I will give you a tip. The answer is simple trig. Have a look at the attached graphic...
You know the value of hh (Hitch Height) and the value of th (Trailer Height). Subtract the two. That gives you the length of one side of a triangle. The distance from the hitch to the center of the trailer's wheels gives you the second side of the triangle. You know know the length of two sides of a triangle. Now you just have to apply simple trig to calculate the angle (theta). Once you get your formula right, then it will work to rotate the trailer to its appropriate position as the vehilce is moving across terrain as well.
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