Question
If $n$ is an odd positive integer and $(1 + x + x^2 + x^3)^n = \sum_{r=0}^{3n} a_r x^r$, then $a_0 - a_1 + a_2 - a_3 + \ldots + a_{3n}$ is equal to ...
If $n$ is an odd positive integer and $(1 + x + x^2 + x^3)^n = \sum_{r=0}^{3n} a_r x^r$, then $a_0 - a_1 + a_2 - a_3 + \ldots + a_{3n}$ is equal to ...