As (1 + 3x + 3x² + x³)²ⁿ = ((1+x)³)²ⁿ

We can write as (1 + x)⁶ⁿ

Here, Total terms are 6n, i.e Even number

Then middle term =

Here, the middle term =

Now,

T_{3n + 1} = ⁶ⁿC_3n

T_{3n + 1} =