determine angle theta for all distances in a movie theater.

In a movie theater the angle theta at which a viewer sees the picture on the screen depends on the distance x of the viewer from the screen. For a movei theater with the dimensions shown in the figure, determine the angle theta (in degrees) for views sitting at distances of 30, 45, 60, 75, and 90 ft. from the screen.

attached a pic, http://img78.imageshack.us/img78/1507/untitledtv3.jpg

tried to draw it as accurate as possible. Can't remember all the geometric rules.

Please help. thanks.