Draw a figure!
There is a point directly under the balloon; it is 200*√3 feet from the tether point, i.e. 346.41 feet.
The height of the balloon is 200 feet.
The 2 observers are equidistant from this point.
The observer points form a right triangle with the balloon and this distance is sqrt(3002-2002)=223.61 feet.
Therefore, one observer is 346.41-223.61 feet from the tether point and the other is 346.41+223.61 feet from the tether point.