The trick to this one is that you know that the two helicopters are flying at the same altitude. You know that the first helicopter is 2km away from the person and its angle of elevation is 30 degrees. With that information you can use one of the trigonometric ratios (sin, cos, or tan) to find the altitude of the first helicopter. Since you know that the two helicopters are flying at the same altitude, you also now know the altitude of the second helicopter. You can use that information and the fact that the second helicopter is 3.5km away from the person to find the second helicopter's angle of elevation by using inverse trigonometric functions (arcsin, arccos, or arctan).
That's a pretty good hint for now. Let's see if you can finish it off from there.