Sorry I keep posting threads - this will likely be my last one. I'm a little stuck as to how to start this problem. If anybody could help me out, I'd appreciate it (I know I've been asking for a lot - you guys are great.)

Two jet airliners, departing from parallel runways, leave Detroit at the same time. The United Airlines jet flies due east towards New York, and the Delta Airlines jet flies due South towards Atlanta. Assuming no air traffic control delays on departure, the jets each reach their cruising altitudes at the same time. At their respective cruise altitudes, the two jets fly at constant speeds with the United Airlines jet flying 50mph faster than the Delta Airlines jet. After flying for 90 minutes, the jets are 870 miles apart. Find each jet's speed in miles per hour.

Okay, so I know that it forms into a triangle, resulting in me having to do the Pythagorean Theorem. I'm just not sure what the sides of the triangle would be. I'm thinking one side would = x and the other 870 + x, but whenever I solve it using those numbers I get a negative number under the radical (using the Quadratic Formula a few steps later.) Any help is appreciated, thanks!