Step 1: Draw a coordinate diagram based on directions.
Let Point B be at coordinates \((0, 0)\).
Then Point A is 4 km South ⇒ \((0, -4)\)
Point C is 3 km East of Point A ⇒ \((3, -4)\)
Step 2: Use distance formula to find BC.
Coordinates of B: \((0, 0)\), Coordinates of C: \((3, -4)\)
\[
\text{Distance} = \sqrt{(3 - 0)^2 + (-4 - 0)^2} = \sqrt{9 + 16} = \sqrt{25} = 5 \text{ km}
\]