The pilot of a helicopter is at an altitude of 10,000 ft and sees a second helicopter at an angle of depression of 30 degrees. the altitude of the second helicopter is 8000 feet what is the distance from the first to the second helicopter along the line of sight?

Consider, an imaginary triangle between those two helicopters, 
Straight distance = 10,000 - 8,000 = 2000 ft
It is base in the triangle, we need to calculate the hypotenuse, 

We know, Cos Ф = B / H
Cos 30 = 2000 / H
√3 / 2 = 2000 / H
H = 2000 * 2 / √3
H = 4000 / √3

In short, Your Answer would be 4000 / √3 or 2313 feet

Hope this helps!