If f(x)=3x+kf(x) = 3x+kf(x)=3x+k and (f∘f)(x)=9x+8(f \circ f)(x) = 9x+8(f∘f)(x)=9x+8, find kkk.
k=1k = 1k=1
k=2k = 2k=2
k=3k = 3k=3
k=4k = 4k=4