Let's make boat speed "b" and the river current speed "c".
210 mi / 10 hr = 21 mph down stream (with current)
21 mph = b + c
210 mi / 70 hr = 3 mph up stream (against current)
3 mph = b - c
The distance both ways is the same. (speed*time=distance)
210 mi = 210 mi
10(b+c) = 70(b-c)
10b +10c = 70b - 70c
80c = 60b
80/60 = b/c
4/3 = b/c
this is the ratio of boat speed to current: b = (4/3)c
21 = b + c
21 = (4/3)c + c
21 = (7/3)c
21(3/7) = c
9 mph = c
3 = b - c
3 = b - 9
12 mph = b