(10. Ria and John started walking at constant speeds from the same spot in opposite directions along the beach. After they had walked for 30 minutes, they were 4km apart.
Rita's speed was 3.5km/h. What was John's speed?​

step-by-step explanation:

points a and b are equidistant from the desired line, so the line you find by using them will go through point r.

using points other than a and b will give the desired line only if they are equidistant from r. if they are randomly located, the perpendicular line you create will not necessarily go through point r, hence will not be a solution to the problem.

d is ur answer hopes this i dont really know how to explain it to u.