The following picture represents an explanation to the given question:
CD represents the beacon
We need to find the distance AB
The measure of the angle C = 90
At the triangle BCD,
The measure of the angle CDB = 90 - 21 = 69
Using the sine law, we will find the length of BD
So,
At the triangle ABC
The measure of the angle CDA = 90 - 10 = 80
So, the measure of the angle ADB = angle CDA - angle CDB = 80 - 69 = 11
At the triangle ADB, using sin law:
substitute with the value of BD and CD s
So,
Rounding the answer to the nearest foot
So, the answer will be AB = 346 ft