Speed of plane a = 425 mph. speed of plane b = 375 mph. both planes leave from kansas city at the same time. plane a flies due west. plane b flies due south. after 3 hours, how far is plane a from plane b?

After 3 hours, Plane A has flown (425 mi/h)*(3 h) = 1275 mi.
After 3 hours, Plane B has flown (375 mi/h)*(3 h) = 1125 mi.

Since the distances flown are in directions that are at a right angle to each other, the Pythagorean theorem can be used to figure the separation. The distance between planes after 3 hours is
  √(1275² +1125²) = √2,891,250 ≈ 1700.4

After 3 hours, Plane A is about 1700 miles from Plane B.