1-The zero at the infinity has no efect in the frequency response . Its is a concept that is good to you know, since you will get across it in some text . for instance you will read that butterworth , chebyshev filters have zeros at infinity. Take a look in any classical filters book and you will see the application of this concept.
2- You dont have to normalise the function, I just did it to show why it is said that the function has zero at infinity .
3-There are a lot of books where you can find good signals and systems theory: Simon Haykin, Oppenheim, Lathi. I have already seen Haykin in Oppenheim in this forum. For root locus , stability and others related, you can look up discrete time control books like Ogata.
4- the set of commands in Matlab are:
>> num=[0,0,12];
>> den=[1,-7,12];
>> sys1=tf(num,den)
Transfer function:
12
--------------
s^2 - 7 s + 12
>> rlocus(sys1).
You can use others to help your analysis, such as:
>> pzmap(sys1)
>> bode(sys1)
The results can lead you to intepret that function has no zero, Matlab interpret zero at infinity as empty set, but if you know the concept you will interpret correctly.
See for example a way to find the function with the zpk comand:
>> z=[];
>> p=[3,4];
>> k=12;
>> sys2=zpk(z,p,k)
Zero/pole/gain:
12
-----------
(s-3) (s-4)
Take care, after all these you should know what z=[] means.
I would like to state clearly that I stand the point : The zero of the given function is at Infinity.