Here, first term = a

Common difference = 3a - a = 2a

Now, Sum of first n terms of an AP is given by:

S_n = [2a + (n - 1)d]

where a is the first term and d is the common difference.

∴ Sum of first n terms of given AP is given by:

S_n = [2a + (n - 1)2a]

= [2a + 2an - 2a]

= [2an]

= n²a