One possible solution is
f(x) = x^4
g(x) = x-3
Since
f(x) = x^4
f(g(x)) = ( g(x) )^4
f(g(x)) = ( x-3 )^4
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Another possible solution could be
f(x) = x^2
g(x) = (x-3)^2
Because
f(x) = x^2
f(g(x)) = ( g(x) )^2
f(g(x)) = ( (x-3)^2 )^2
f(g(x)) = (x-3)^(2*2)
f(g(x)) = (x-3)^4