Question 848461
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Hi,
Let x represent the speed of the canoe in still water
{{{18mi/(x+3mph) + 18mi/(x-3mph) = 8hr}}}  |Multiplying thru by (x+3mph)(x-mph3) so as all denominators = 1
    36x = 8x^2 - 72
  8x^2 -36x - 72 = 0
  2x^2 -9x - 18 = 0
{{{x = (-b +- sqrt( b^2-4*a*c ))/(2*a) }}} 
{{{x = (9+- sqrt( 225))/(4) }}} 
 {{{x = (9+- 15)/(4) }}}   (Tossing out the negative solution for unit measure)
  x = 6mph is the speed of the canoe in still water
CHECKING our answer***
{{{18mi/(9mph) + 18mi/(3mph)}}} = 8hr
Wish You the Best in your Studies.