#### Answer

The Yangtze River is longer by 29 km.

#### Work Step by Step

We know that 1 mile = 1.6 kilometers.
We can convert the length of the Yangtze River from units of miles to units of kilometers.
$3940~mi = \frac{3940~mi}{1}\times \frac{1.6~km}{1~mi} = 6304~km$
The Yangtze River has a length of 6304 km.
The Mississippi River has a length of 6275 km.
The Yangtze River is longer by 29 km.