Help me ASP PLEAS

Which of the following is the explicit rule for a geometric sequence defined a recursive formula of a -5a for which the first term is 23?

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).

From the beginning of the day the temperature rose 15 degrees so that by 4 pm it was 87 degrees. what was the temperature at the start of the day