When the boat goes downstream, its speed relative to the river's bank is the sum of the boat rate in still water and the current rate.


When the boat goes upstream, its speed relative to the river's bank is the difference of the boat rate in still water and the current rate.


This it the key for solving "Upstream-Downstream" Travel and Distance problems.


So, your equations are

u + v = 13,    (1)    (u is the boat speed in still water,
u - v =  3.    (2)     v is the currebt speed.)

Add the equations (1) and (2). You will get

2u = 13 + 3 = 16.   Hence,  u =  = 8 kph.

Then you can easily find v from (1): v = 13-8 = 5.

Answer.  The current speed is 5 kph.  The boat speed in still water is 8 kph.