Question 388069
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Hi
Let x and (x-3) represent Elvira's and David's times respectively
D = r*t  
Distances traveled are equal in this example***
 14mph*x = 21mph(x-3)
solving for x
 14x = 21x - 63
  63 = 7x
x = 9hr, the time that Elvira traveled.  David traveled 6hrs   (9hr-3hr = 6hr)

CHECKING our Answer***
 14mph*9hr = 126mi = 21mph*6hr = 126mi