Answer:
actually what is this is you is this a geometry, trigonometry I don't know what to call
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
Square root of 6 times 16 will give us 39.19
The square root of 48 times 32 [which gives us 1536] is also equal to 39.19
The square root of 1536 is equal to 39.19
Hence, x is equal to 32
Hope this helped! :D