Your picture has the observer on the ground moving forward (at 550 mph?!). The observer is stationary and it is the jet that is moving. Your picture should have the two lines, at 15 and 67 degrees, together at the bottom and apart at the top.
Since the plane is flying at 550 mi per hour, in one minute it will fly 550/60= 9 and 1/6 miles.
That will be the length of the line connecting those two lines at altitude h.
