∵2^（n+2）=a ∴4×2^n＝a ∴2^n＝a/4
∴4^(3m-n)＝2^[2×(3m－n)]＝2^(6m)／2^(2n)＝[(2^m)^6]/[(2^n)^2]＝[(a^2)^6]/(a/4)²
＝16×a^12/a²＝16a^10