9的m次幂乘27的m-1次幂除以3的2m次幂
= 3的2m次幂 ×3的3(m-1)次幂 ÷ 3的2m次幂
= 3的[2m + 3(m-1) - 2m]
= 3的3m-3次幂 = 27 = 3的3次幂
因此3m-3 = 3
M = 2