Show me how to use a proportion demonstrating the side splitter theorem

This is different from the first one can someone me

If the ratio of sum of the first m and n terms of an ap is m2 : n2 , show that the ratio of its mth and nth terms is (2m − 1) : (2n − 1).

Adam used 1072.4 gallons of water over the course of 7 days. how much water did adam use, on average, each day?