Find the missing length of the triangle.

To solve these type of problems you need to use the pythagoras theorem ⇨ Hypotenuse² = Base² + Altitude².

Here,

Now, let's solve for c.

Hypotenuse² = Base² + Altitude²

c² = (20)² + (21)²

c² = 400 + 441

c² = 841

c = √841

c = 29.

So, the value of c is 29 km.