hey i try to solve a task, but i dont get it

Compute the coefficient repr. of (p(x))² by using FFT algorithm.

p(x) = 2x^3 - x² + 4x + 1

this is the way i wanted to started(i dont know if I am right)

i have a input with length N = 4 its Array a [2, -1, 4, 1]

then i split them in odd and even

d[0] = FFT([-1,1])

d[1] = FFT([2, 4])

i dont know how to proceed further, every help would be appreciated