Question 636753
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Hi,
Yes. speed of boat against stream x-3
Yes. speed of boat with stream x+3 
Note: D = rt  0r  {{{t = D/r}}}
Question states*** The round trip takes 5 hours
{{{36/(x-3) + 36/(x+3) = 5hr}}} |Multiplying thru by (x-3)(x+3) so as all denominators = 1 
   36(x+3) + 36(x-3) = 5(x-3)(x+3)
         72x = 5(x^2 - 9)
        5x^2 - 72x - 45 = 0
factor: (5x + 3)(x -15) = 0 , 
             x = 15mph, speed of the boat in still water (throwing out negative solution for unit measure)