f(x)=kx+m
f(kx+m)=k(kx+m)+m=k^2x+km+m=4x-1
k^2=4 k=2 k=-2
km+m=-1 m=-1/3 m=1
f(x)=2x-1/3 f(3)=17/3
f(x)=-2x+1 f(3)=-5