Alexa's friends got her a skydiving lesson for her birthday. her helicopter took off from the skydiving center, ascending in an angle of 20 degrees, and traveled a distance of 3.4 kilometers before she fell in a straight line perpendicular to the ground.

I'm making the that the unknown is the vertical distance fallen.  In the right triangle, the missing side (vertical distance) is opposite the given angle.  3.4 km is the hypotenuse, since it is the distance the helicopter traveled as it ascended.  We then have opposite/hypotenuse, which is the ratio for sine.
sin 20 = x/3.4
Multiply by 3.4:
3.4sin20 = x
This gives us 3.1 for the vertical distance.
If, however, you need to know the horizontal distance from the starting point, we would use cosine, since that leg is adjacent to the given angle:
cos 20 = x/3.4
3.4cos 20 = x
1.4 = x
The helicopter comes down 1.4 kilometers away from where it lifted off.