Filter function in matlab

I need to get filter function using coefficents of notch filter.If transfer function of notch filter is

        1-2rcos(w)z^-1+r^2z^-2
H(z)= --------------------------     w=notch frequency will be 50 hertz
         1-2cos(w)z^-1+z^-2

How can I use this transfer function with filter function lets say our input signal

t=0:dt:0.2;
x=cos(2*pi*25*t)+cos(2*pi*50*t);

I want to filter this input signal with filter function

y=filter(b,a,x);

Lastly how can I draw magnitude spectrum of this filter with freqz if notch frequency is 50 hertz?