If the straight lines are parallel the distance between r and s is the distance from a point of any of both straight lines to any other. For a line segment you should create a right triangle by making two more lines. Measure by clicking multiple times on the map or add locations above.
Enter a city a zipcode or an address in both the Distance From and the Distance To address inputs.
Then find the distance between the units of the points which is 12 and the distance between the points which is 5. When using Google Maps in a desktop web. If this line segment is on a dot grid every line between two dots is one unit. Click Calculate Distance and the tool will place a marker at each of the two addresses on the map along with a line between them.