Solution:

$N=m_{1} N_{1}$

$m_{1}=2 \quad N_{1}=3$

$x(k)=\sum_{n=0}^{N_{1}-1} x\left(n m_{1}\right) \ W_N^{nm_1k \ N_1 - 1} + \sum^{N_1 - 1}_{n = 0} \ x(nm_1 + 1)^{(nm_1 + 1) k}_{W_N}$

$+\cdots+\sum^{N_1 - 1}_{n = 0} \ x(nm_1 + (m_1 - 1))^{nm_1 + (m_1 - 1)) \ k }_{W_N}$

$x(k)=\sum_{n=0}^{2} x(2 n) w_{6}^{2nk}+\sum_{n=0}^{2} …