I just need to prove that 1 is in the range of f.
Some work of mine may help you to help me..[3]
1) f(0) = 0
2) f(f(m)) = f(m) for all m
3) f(m+ f(n)) = f(m) + f(n)= f(f(m)+ f(n))
4) f(k(f(m)) = kf(m)
5)If 1 is in range of f we must have f(1) = 1
So as you can see, we just need to prove f(1) = 1 to get the non-constant solution or f(1) = 0 to get constant solution.
But thats the step where i am stuck...[2]