Let's think of the line segment KM as a road. It starts at point K and ends at M. You start driving from point K and after 4 miles, you have reached point L. The segment KL is equal to 4 because the distance between K and L is 4.
From point L to point M, you drive 11 miles. You are now at the end of the road. We want to find out how long the road is.