They're just asking for the length of each heavy line !
For each one, you can easily see how many boxes TALL it is, and how many boxes WIDE it is. Those two measurements, plus the heavy line, make a right triangle. The two measurements are the legs of the triangle, and the heavy line is the hypotenuse. So the length of the heavy line is
√(one measurement² + the other measurement²).
For example, the first one, A+B+C .
Line A is 2 boxes to the RIGHT and 2 boxes UP.
Line B is 2 boxes the LEFT and 3 boxes DOWN.
Line C is 1 box to the RIGHT and 2 boxes DOWN.
When you addum up, you get 1 to the RIGHT and 3 DOWN.
Length = √(1²+3²)
Length = √(1 + 9)
Length = √10 boxes
But at the beginning, they said that each box has edges of 10 km.
So the length is (10km) x (√10).
That's about 31.6 km.