Let x be the rate of the boat in still water.


Then  the effective rate rowing downstream is (x+2) mph,

while the effective rate rowing upstream   is (x-2) mph.


Time rowing 42 miles downstream is    hours;

Time rowing 42 miles upstream   is    hours.


Time equation, according to the problem, is


     -  = 8   hours.


To solve this equation, multiply both sides by  (x-2)*(x+2)  and simplfy.


    42(x+2) - 42(x-2) = 8*(x-2)*(x+2)

    42x + 84 - 42x + 84 = 8*(x^2-4)

           168          = 8(x^2-4)

           21           =   x^2 - 4

           21 + 4       = x^2

           x^2          = 25

            x           =  = 5.


ANSWER.  The boat's rate in still water is 5 miles per hour.