An airplane is flying at a constant speed and altitude, on a line that will take it directly over a radar station located on the ground. At the instant the airplane is 60,000 feet from the station, an observer in the station notes that its angle of elevation is 30 degrees and is increasing at a rate of 0.5 degrees per second. Find the speed of the airplane.

So I know this involves taking the derivative, but I can't formulate a general equation that relates the known variables: the distance from the station (x), the angle of elevation ( $\displaystyle \Theta$ ), the rate that the angle is increasing( $\displaystyle d \Theta /dt $), and the speed ($\displaystyle dx/dt$ ).