If Sunil rows at 5 km/h in still water and at 3.5 km/h against the current, then Sunil's speed with the current is:

Explanation

Still-water speed is 5 km/h and upstream speed is 3.5 km/h, so current speed is 1.5 km/h. Downstream speed is 5 + 1.5 = 6.5 km/h.