Let R=the rate going

Make this chart:

             Distance      Rate      Time
Going                     
Returning                                        

Fill in the two distances which is given as 7 each:
Fill in R for the rate going:


             Distance      Rate      Time
Going           7           R                 
Returning       7        

We are told that her rate returning was 2 mph less than 
her rate going, so subtract 2 from R, getting R-2, and
fill that in for her rate returning:

             Distance      Rate      Time
Going           7           R                
Returning       7          R-2       


Now we use the fact that  
to fill in the two TIMES:


             Distance      Rate      Time
Going           7           R        7/R          
Returning       7          R-2       7/(R-2)

We are told that she rowed her boat across Lake Bend and 
back in 3 hours, so we make our equation from that by
adding the two times and setting that equal to 3 hours.



Multiply by the LCD of 











That doesn't factor so we have to use the quadratic
formula:



with , , , 















Approximating the two solutions as decimals,

R=5.871924369 and R=0.794742298

We have to discard the second answer, since that would
make her rate returning, , a negative number.

So the answer is about 5.87 mph. going (and about 3.87 mph
returning).

Edwin