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
不懂的欢迎追问,