Answer:
c = 
 km
Step-by-step Step-by-step explanation:
Here's the required formula to find the missing side of triangle by pythagoras theorem :

 a = 20 km
 b = 21 km
 c = ?
Substituting all the given values in the formula to find the third side of triangle :

Hence, the length of missing side of triangle is 29 km. 
