Finding the rate at which D is changing with respect to theta when theta=45 degrees?

A searchlight is trained on the side of a tall building. As the light rotates, the spot is illuminates moves up the and down the side of the building. That is, the distance D between ground level and the illuminated spot on the side of the building is a function of the angle theta formed by the light beam and the horizontal. If the searchlight is located 50 m from the building, find the rate at which D is changed with respect to theta when theta=45 degrees. Express your answer in units of meters/degrees.