a^(n+1)bⁿ -4a^(n+2)+3abⁿ -12a²
=[a^(n+1)bⁿ+3abⁿ]-[4a^(n+2)+12a²]
=abⁿ(aⁿ+3)-4a²(aⁿ+3)
=a(aⁿ+3)(bⁿ-4a)