Ok, so let's call the distance of each of the rivers "X". The shorter river is length X, while the longer river is X +261. We know the total of the 2 rivers is 12,875 so we can set up our equation like this:
X + (X + 261) = 12,875
If we subtract 261 from each side, we get this equation:
X + X = 12,614
Combining X's we get:
2X = 12,614
And if we divide both sides by 2 to get the X by itself we get:
X = 6,307
So our shorter river is 6,307 km and our longer river is 6,307 + 261 = 6,568 km.
I'm not exactly sure what grade level this is, but I've linked some pre-algebra word problems below which are similar and could help. Good luck!