One leg of the triangle is the vertical line which as a constant 6 mile length. The other leg is the horizontal line of flight of the airplane and you are not given its length (but can calculate at the given instant). The length of the hypotenuse is the distance to the airplane, 10 miles and is increasing at 270 mph. (I suspect that there is a picture in your textbook since you say " the airplane is 10 miles away (s = 10)" without telling us what "s" is!).
Set up the Pythagorean theorem from that and differentiate with respect to time. Solve for the rate at the length of the horizontal leg is changing.
Try that and if you have trouble, come back and show what you have done.