设f(x)=ax+b,a不等于0
则f(f(x))=f(ax+b)=4x+6
令x=(y-b)/a,则f(y)=4(y-b)a+6=ay+b
故4/a=a,6-4b/a=b
a=2,b=2或a=-2,b=-6
故f(x)=2x+2或-2x-6