a^n=1/2
b^2n=3
(-a^2b)^4n
=[(-a^2b)^4]^n
=[(-1)^4(a^2b)^4]^n
=1^n*(a^2b)^4n
=a^8n*b^4n
=(a^n)^8*(b^2n)^2
=(1/2)^8*3^2
=9/256