If the first, second and last term of an A.P. are a, b and 2a respectively, its sum is
a, b and 2a are in A.P so a is the first term and 2a is the last term denoted by T and Tn respectively. Here Common difference = b – a
Tn = 2a = a + (n–1) (b–a)
So n = 
Sum = 
{first term + last term}
=
{ 3a }
= 
12