are you sure that's the answer? that's not what i got. at least, did they tell you how high the plane was flying?

the first thing you have to do is "convert" 100m to radians, so instead of m/sec you have rad/sec. so in one second, the plane covers 100m, now recall that , where is the arclength, is the radius and is the angle that subtends the arc in radians. since they didn't tell you how high the plane is, you must use km. then we have km, and so :