(3x+n)^2=9(x^2)+mx+36
9(x^2)+6nx+n^2=9(x^2)+mx+36
(6n-m)x+n^2-36=0
因为对任意x恒成立
6n-m=0n^2-36=0
n=6或-6
m=36或-36
m-n=30或-30