at maximum speed, an airplane travels 2100 miles against the wind in 6 hours. flying with the wind, the plane can travel the same distance on 4 hours

Plane's air speed is 437.5 mph 
Wind velocity is 87.5 mph  
An actual question hasn't been asked, but an educated guess would be to determine the plane's airspeed or the wind velocity. So I'll calculate those values for you.  
When traveling with the wind, the combined speed is 2100/4 = 525 mph.
When traveling against the wind, the combined speed is 2100/6 = 350 mph.
So we can create 2 equations with 2 unknowns. They are:
 a - b = 350
 a + b = 525 
 Let's add the two equations together.
 a - b = 350
 a + b = 525
 2a = 875
 a = 437.5 
 So the plane's speed in still air is 437.5 mph.
 Let's calculate the wind velocity.
 a + b = 525
 437.5 + b = 525
 b = 87.5
 And the wind velocity is 87.5 mph