First, multiply 50*3 to get the answer in feet which is 150 ft.
Then, divide 150 ft by 60 which would be 2.5 ft per second.
plane 1 travels 395 miles in 1 hour
plane 2 travels 422 miles in 1 hour
together, they travel (395+422) 817 miles in 1 hour
to have 2451 miles between then, they'd need to travel

so in 3 hours, they'll be 2451 miles apart