We have the following problem:
We know that x=25° and y=55°, therefore we can know the value of alpha, since

We also notice that since the higway is straight and the plane is flying in an horizontal path this lines area parallel, then

this follows from the fact that this angles are alternate with each other.
Once we know the values of the angles we are prepared to find the distance from tha plane to point A. Using the Law of sines we know that

where, in this case, a=6 mi and c is the distance we are looking for. Plugging the values we know and solving for c we have

Therefore the distance from the plane to point A is 4.99 mi.
Any questions so far?