can someone explain this please

An air traffic controller spots two planes flying at the same altitude. Their flight paths form a right angle at point P. One plane is 150 miles from point P and is moving at 420 miles per hour. The other plane is 200 miles from point P and is moving at 420 miles per hour. Write the distance s between the planes as a function of time t. s (t) = 200- Distance (in miles) 100 - 100 200 Distance (in miles)