Question 1094371
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Let x be the unknown speed of the boat in still water.


Then  the effective speed upstream    is x-2  miles per hour,

while the effective speed downstream  is x+2  miles per hour.


The time spent to cover 18 miles upstream is {{{18/(x-2)}}},

while the time to cover 21 miles downstream is {{{21/(x+2)}}}.


Your time equation is 

{{{18/(x-2)}}} + {{{21/(x+2)}}} = 4.


To solve it, multiply both sides by (x-2)*(x+2). You will get

18*(x+2) + 21*(x-2) = 4*(x^2-4).


Simplify:

18x + 36 + 21x - 42 = 4x^2 - 16,

4x^2 - 39x + 10 = 0.

Apply the quadratic formula and find the solution x = 10 mph.
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