For a certain arithmetic progression, the sum of the first 2n terms is three times the sum of its first n terms.
a. (i) Show that 2a=(n+1)d, where a is the first term and d is the common difference.
b. (ii) If n=5 and the 5th term is 21, find the values of a and d.
c. (iii) Using the values obtained in (ii), determine the sum of the first 15 terms of the progression.