Unit impulse response of a cascade interconnection of three discrete-time systems I am nearly at the end of finishing a problem in my textbook but I couldn't understand something in the answer;

I did everything to the point I found the overall response of the system in terms of $h_{1}[n]$ which is $$h[n]=h_{1}[n]+2h_{1}[n-1]+h_{1}[n-2]$$ Now while comparing each $n$ value with the given graph for overall response, the answer is saying that for $n=0$;  $$h[0]=h_{1}[0]$$ but I didn't get why it isn't like this: $$h[0]=h_{1}[0]+2h_{1}[-1]+h_{1}[-2]$$ How do we know $h_{1}[n]$ is zero for $n$ values smaller than zero?
HINT:

Consider the cascade interconnection of three causal LTI systems, [...]&quot;

What does that imply for the impulse response $h_1[n]$?
What is more, since the total impulse response is causal, and you know from its definition that $h_2[n]$ is causal, $h_1[n]$ must also be causal.