a(x^my^4)/(3x^2y^n)^2
=a(x^my^4)/(9x^4y^2n)
=(a/9)*x^(m-4)*y^(4-2n)
=4x^2y^2
所以a/9=4,m-4=2,4-2n=2
所以a=36,m=6,n=1