let

v = Abby's speed in still water, v>0

d = 10 km the distance Abby is crossing in each direction

t = 3 hr the total time

c = 5 km/h the speed of the river

going downstream Abby's speed is: v + c = v + 5

going upstream Abby's speed is: v - c = v - 5

since speed = distance/time follows time = distance/speed

d/(v + c) + d/(v - c) = t

10/(v + 5) + 10/(v - 5) = 3

by solving the above system of equations we find:

v = 9.34 km/h

click here to see the step by step solution of quadratic equation:

Abby's speed in still water is 9.34 km/h approximately.