若n为正整数,且x的2n次方=7,求(3x的3n次方)的平方-13(X的平方)的2n次方得值.

已知：x^(2n)=7求：3*[(x)^(3n)]^2-13*(x^2)^(2n)=?3*[(x)^(3n)]^2-13*(x^2)^(2n)=3*x^(6n)-13*x^(4n)=3*[x^(2n)]^3-13*[(x^(2n)]^2=3*7^3-13*7^2=21*7^2-13*7^2=8*7^21=8*49=392